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{{otheruses6|American football (disambiguation)|Football}}
{{sport
| image=Titans Texans.jpg
| imagesize=300px
| caption=American football game between the [[Tennessee Titans]] and the [[Houston Texans]] in 2005.
| union=
| nickname=Football, [[Gridiron football]]
| first=November 6, 1869, [[Rutgers University]] vs. [[Princeton University]]
| registered=
| clubs=
| largest organization= [[National Football League]]
| team=11 at a time
| mgender=
| category=
| ball=[[Football (ball)|Football]]
| olympic=
}}

'''American football''', known in the [[United States]] and [[Canada]] simply as '''football''',<ref>In the United States and Canada, the term "football" may refer to either American football or to the similar sport of [[Canadian football]], the meaning usually being clear from the context. This article describes the American variant.</ref> is a competitive [[team sport]] known for mixing strategy with physical play. The objective of the game is to score points by advancing the [[football (ball)#American and Canadian football|ball]]<ref>Technically, the ball is a [[prolate spheroid]]. ''See'' [http://www.ncaa.org/library/rules/2006/2006_football_rules.pdf 2006 NCAA Football Rules and Interpretations, Sec. 1, Art. 1]</ref> into the opposing team's [[end zone]]. The ball can be advanced by carrying it (a running play) or by throwing it to a teammate (a passing play). Points can be scored in a variety of ways, including carrying the ball over the opponent's goal line; catching a pass from beyond that goal line; [[placekicker|kicking]] the ball through the goal posts at the opponent's end zone; and tackling an opposing ballcarrier within his end zone. The winner is the team with the most points when the time expires.

The sport is also played outside the United States. National leagues exist in [[German Football League|Germany]], [[Italy]], [[Switzerland]], [[Vaahteraliiga|Finland]], [[Sweden]], [[X-League|Japan]], [[Mexican College Football|Mexico]], [[Israel]], [[LNFA|Spain]], [[Austria]] and several Pacific Island nations. The [[National Football League]], the largest professional American football league in the world, ran a [[NFL Europa|developmental league]] in Europe from 1991–1992 and then from 1995–2006.

American football is closely related to [[Canadian football]], [[comparison of Canadian and American football|but with significant differences]]. Both sports originated from [[rugby football]].

==History==
{{main|History of American football}}
[[Image:Unknown Early American Football Team.jpg|thumb|right|230px|An early American football team, from the turn of the twentieth century]]

The history of American football can be traced to early versions of [[rugby football]] and [[association football|soccer]]. Both games have their origins in [[football|varieties of football]] played in the [[United Kingdom]] in the mid-19th century, in which a [[football (ball)|ball]] is kicked at a [[Goal (sport)|goal]] and/or run over a line. Also like soccer, American football has twenty two players on the field of play. Furthermore, some player position references from soccer are used, such as the term "halfback" and "fullback".

[[Image:Walter Camp - Project Gutenberg eText 18048.jpg|thumb|left|Walter Camp]]American football resulted from several major divergences from [[rugby football]], most notably the rule changes instituted by [[Walter Camp]], considered the "Father of American Football". Among these important changes were the introduction of the [[line of scrimmage]] and of [[Down (football)|down-and-distance]] rules. In the late 19th and early 20th centuries, gameplay developments by college coaches such as [[Eddie Cochems]], [[Amos Alonzo Stagg]], [[Knute Rockne]], and [[Glenn Scobey Warner|Glenn "Pop" Warner]] helped take advantage of the newly introduced [[forward pass]].

The popularity of [[College football|collegiate football]] grew as it became the dominant version of the sport for the first half of the twentieth century. [[Bowl game]]s, a college football tradition, attracted a national audience for collegiate teams. Bolstered by fierce [[List of NCAA college football rivalry games|rivalries]], college football still holds widespread appeal in the US.<ref name=PFRA2> {{cite web | title = Camp and His Followers: American Football 1876–1889 | work = The Journey to Camp: The Origins of American Football to 1889 | publisher = Professional Football Researchers Association | url = http://www.footballresearch.com/articles/frpage.cfm?topic=d-to1889 | accessdate = 2007-05-16}}</ref><ref name=histfoot>{{cite web | title = The History of Football | work = The History of Sports | publisher = Saperecom | year = 2007 | url = http://www.historyoffootball.net/ | accessdate = 2007-05-15}}</ref><ref name=NFL1869> {{cite web | title = NFL History 1869–1910 | work = NFL.com | publisher = NFL Enterprises LLC | year = 2007 | url = http://www.nfl.com/history/chronology/1869-1910 | accessdate = 2007-05-15}}</ref>

The origin of professional football can be traced back to 1892, with [[William Heffelfinger|William "Pudge" Heffelfinger's]] $500 contract to play in a game for the Allegheny Athletic Association against the Pittsburgh Athletic Club. In 1920 the American Professional Football Association was formed. The first game was played in Dayton, Ohio on October 3rd, 1920 with the host Triangles defeating the Columbus Panhandles 14-0. The league changed its name to the [[National Football League]] (NFL) two years later, and eventually became the [[Major North American professional sports leagues|major league]] of American football. Initially a sport of Midwestern, industrial towns in the United States, professional football eventually became a national phenomenon. Football's increasing popularity is usually traced to the [[1958 NFL Championship Game]], a contest that has been dubbed the "Greatest Game Ever Played". A rival league to the NFL, the [[American Football League]] (AFL), began play in 1960; the pressure it put on the senior league led to a [[AFL-NFL merger|merger]] between the two leagues and the creation of the [[Super Bowl]], which has become the most watched television event in the United States on an annual basis.<ref name=popular>{{cite web | url = http://www.coldhardfootballfacts.com/Documents/NFL_all_about_SB_1-07.pdf | title = NFL:America's Choice | year = 2007 | publisher = National Football League | accessdate = 2007-08-15 | format = [[PDF]]}}</ref>

==Rules==
{{main|American football rules}}
The object of American football is to score more points than the opposing team within the time limit.

===Field and players===
[[Image:AmFBfield.svg|thumb|right|250px|The numbers on the field indicate the number of [[yard]]s to the nearest end zone.]]

American football is played on a field {{convert|360|by|160|ft|1}}. The longer boundary lines are ''sidelines'', while the shorter boundary lines are ''end lines''. Near each end of the field is a ''goal line''; they are {{convert|100|yd|sigfig=3}} apart. A scoring area called an ''[[end zone]]'' extends {{convert|10|yd|sigfig=2}} beyond each goal line to each end line. While the playing field is effectively flat, it is common for a field to be built with a slight crown—with the middle of the field higher than the sides—to allow water to drain from the field.

''Yard lines'' cross the field every {{convert|5|yd}}, and are numbered every 10 yards from each goal line to the 50-yard line, or midfield (similar to a typical [[rugby league]] field). Two rows of short lines, known as inbounds lines or ''hash marks'', run parallel to the sidelines near the middle of the field. All plays start with the ball on or between the hash marks.

At the back of each end zone are two ''[[goalpost]]s'' (also called ''uprights'') connected by a crossbar {{convert|10|ft|m|sigfig=3}} from the ground. The posts are, for high skill levels {{convert|222|in|m|sigfig=3}} apart. For lower skill levels, these are widened to {{convert|280|in|m|sigfig=3}}.

Each team has 11 players on the field at a time. However, teams may substitute for any or all of their players, if time allows, during the break between plays. As a result, players have very specialized roles, and, sometimes (although rarely) almost all of the (at least) 46 active players on an NFL team will play in any given game. Thus, teams are divided into three separate units: the [[offensive team|offense]], the [[defensive team|defense]] and the [[special teams]].

===Start of halves===
Similar to association football, the game begins with a [[Coin flipping|coin toss]] to determine who will kick off to begin the games and which goal each team will defend. The options will be presented again to start the second half; the choices for the first half do not automatically determine the start of the second half. The referee will conduct the coin toss with the captains (or sometimes coaches) of the opposing teams. The team that wins the coin toss has three options:
#They may choose to either receive the opening kickoff or to kick off.
#They may choose which goal to defend.
#They may choose to ''defer'' the first choice to the other team and have first choice to start the second half.<ref>{{cite web | url = http://ap.google.com/article/ALeqM5gHSwbXq1wnH-jmEl-jV4W8M0Vd1wD8VQBFSG0| title=NFL Makes Some Rule Changes | year = 2008 | accessdate = 2008-04-03}}</ref>
Whatever the first team chooses, the second team has the option on the other choice (for example, if the first team elects to receive at the start of the game, the second team can decide which goal to defend).

At the start of the second half, the options to kick, receive, or choose a goal to defend are presented to the captains again. The team which did not choose first to start the first half (or which deferred its privilege to choose first) now gets first choice of options.<ref name=NCAArules>{{cite web | url = http://www.ncaa.org/library/rules/2005/2005_football_rules.pdf | title = 2005 Rules and Interpretations | publisher = National Collegiate Athletic Association | year = 2005 | accessdate = 2008-01-09}}</ref>

===Game duration===
A standard football game consists of four 15-minute quarters (12-minute quarters in high-school football and often shorter at lower levels), with a [[half-time]] intermission after the second quarter. The clock stops after certain plays; therefore, a game can last considerably longer (often more than three hours in real time), and if a game is broadcast on [[television]], [[Television timeout|TV timeouts]] are taken at certain intervals of the game to broadcast [[advertising|commercials]] outside of game action. If an NFL game is tied after four quarters, the teams play an additional period lasting up to 15 minutes. In an NFL overtime game, the first team that scores wins, even if the other team does not get a possession; this is referred to as [[sudden death (sport)|sudden death]]. In a regular-season NFL game, if neither team scores in overtime, the game is a tie. In an NFL playoff game, additional overtime periods are played, as needed, to determine a winner. College overtime rules are more complicated and are described in [[Overtime (sport)]].

[[Image:NSU Football.jpg|thumb|left|A [[line of scrimmage]] on the 48-yard line. The offense is on the left.]]
[[Image:CAL football (edited by CPacker).jpg|thumb|left|A [[quarterback]] searching for opportunity to throw a pass.]]
[[Image:2006 Pro Bowl tackle.jpg|thumb|left|A [[running back]] being tackled when he tries to run with the ball.]]
[[Image:Shea Smith.JPG|thumb|left|A quarterback passing.]]
[[Image:Orton To Wolfe.jpg|thumb|left||Forward pass in progress, during practice.]]
[[Image:Alabama Field-Goal.JPG|thumb|left|A kicker attempts an extra point.]]

===Advancing the ball===
Advancing the ball in American football resembles the ''six-tackle rule'' and the ''play-the-ball'' in [[rugby league]]. The team that takes possession of the ball (the '''offense''') has four attempts, called '''[[Down (football)|downs]]''', to advance the ball {{convert|10|yd|m|1}} towards their opponent's (the '''defense'''<nowiki>'</nowiki>s) end zone. When the offense gains 10 yards, it gets a '''first down''', which means the team has another set of four downs to gain yet another 10 yards or score with. If the offense fails to gain a first down (10 yards) after 4 downs, the other team gets possession of the ball at the spot of the football, beginning with their first down.

Except at the beginning of halves and after scores, the ball is always put into play by a '''[[Snap (American football)|snap]]'''. Offensive players line up facing defensive players at the [[line of scrimmage]] (the position on the field where the play begins). One offensive player, the [[Center (football)|center]], then passes (or "snaps") the ball between his legs to a teammate, usually the [[quarterback]].

Players can then advance the ball in two ways:
:::#By running with the ball, also known as '''[[Rush (football)|rushing]]'''. One ball-carrier can hand the ball to another player or throw backwards to another player. These are known as a '''handoff''' and a '''backward pass''' (sometimes referred to as a '''[[Lateral pass|lateral]]''') respectively.
:::#By throwing the ball to a teammate, known as a [[forward pass]] or as '''passing''' the football. The forward pass is a key factor distinguishing American and Canadian football from other football sports. The offense can throw the ball forward only once during a down and only from behind the line of scrimmage. The ball can be thrown, pitched, handed-off, or tossed sideways or backwards at any time.

A down ends, and the ball becomes dead, after any of the following:
:::*The player with the ball is forced to the ground (tackled) or has his forward progress halted by members of the other team (as determined by an [[Official (American football)|official]]).
:::*A forward pass flies beyond the dimensions of the field ('''out of bounds''') or touches the ground before it is caught. This is known as an '''[[incomplete pass]]'''. The ball is returned to the most recent line of scrimmage for the next down.
:::*The ball or the player with the ball goes out of bounds.
:::*A team scores.

Officials blow a whistle to notify players that the down is over.

Before each down, each team chooses a '''play''', or coordinated movements and actions, that the players should follow on a down. Sometimes, downs themselves are referred to as "plays."

===Change of possession===
The offense maintains possession of the ball unless one of the following things occurs:
*The team fails to get a first down&mdash; i.e., in four downs they fail to move the ball past a line 10 yards ahead of where they got their last first down (it is possible to be downed behind the current line of scrimmage, "losing yardage"). The defensive team takes over the ball at the spot where the 4th-down play ends. A change of possession in this manner is commonly called a '''turnover on downs''', but is not credited as a defensive "turnover" in official statistics. Instead, it goes against the offense's 4th down efficiency percentage.
*The offense scores a touchdown or field goal. The team that scored then kicks the ball to the other team in a special play called a '''[[Kickoff (American football)|kickoff]]'''.
*The offense punts the ball to the defense. A '''[[punt (football)|punt]]''' is a kick in which a player drops the ball and kicks it before it hits the ground. Punts are nearly always made on fourth down (though see ''[[quick kick]]''), when the offensive team does not want to risk giving up the ball to the other team at its current spot on the field (through a failed attempt to make a first down) and feels it is too far from the other team's goal posts to attempt a field goal.
*A defensive player catches a forward pass. This is called an '''[[interception]]''', and the player who makes the interception can run with the ball until he is tackled, forced out of bounds, or scores.
*An offensive player drops the ball (a '''[[fumble]]''') and a defensive player picks it up. As with interceptions, a player recovering a fumble can run with the ball until tackled, forced out of bounds, or scores. Backward passes that are not caught do not cause the down to end like incomplete forward passes do; instead the ball is still live as if it had been fumbled. Lost fumbles and interceptions are together known as '''turnovers'''.
* The offensive team misses a field goal attempt. The defensive team gets the ball at the spot where the previous play began (or, in the NFL, at the spot of the kick). If the unsuccessful kick was attempted from within {{convert|20|yd|m|1}} of the end zone, the other team gets the ball at its own 20 yard line (that is, 20 yards from the end zone). If a field goal is missed and the ball remains in the field of play, a defensive player may also catch the ball and attempt to advance it.
*In his own end zone, an offensive ballcarrier is tackled, forced out of bounds or loses the ball out of bounds, or the offense commits certain fouls. This fairly rare occurrence is called a '''[[Safety (football score)|safety]]'''.
*An offensive ballcarrier fumbles the ball forward into the end zone, and then the ball goes out of bounds. This extremely rare occurrence leads to a '''touchback''', with the ball going over to the opposing team at their 20 yard line (Note that touchbacks during non-offensive special teams plays, such as punts and kickoffs, are quite common).

===Scoring===
A team scores points by the following plays:
* A '''[[touchdown]]''' (TD) is worth 6 points. It is scored when a player runs the ball into or catches a pass in his opponent's end zone. A touchdown is analogous to a [[try]] in rugby with the major difference being that a try requires the player to place the ball on the ground.
** After a touchdown, the scoring team attempts a [[Try (American Football)|try]] (which is also analogous to the [[Try#Conversion|conversion in rugby]]). The ball is placed at the other team's {{convert|3|yd|m|1|sing=on}} line (the {{convert|2|yd|m|1|sing=on}} line in the NFL). The team can attempt to kick it over the crossbar and through the goal posts in the manner of a field goal for 1 point (an '''[[extra point]]''' or '''point-after touchdown (PAT)<ref>{{cite book | last = | first = | authorlink = | coauthors = | title = 2007 Official Rules of the NFL | publisher = Triumph Books |date=1 October 2007 | location = | pages = | url = | doi = | id = | isbn = 1699780288}}</ref>'''), or run or pass it into the end zone in the manner of a touchdown for 2 points (a '''[[two-point conversion]]'''). In college football, if the defense intercepts or recovers a fumble during a one or two point conversion attempt and returns it to the opposing end zone, the defensive team is awarded the two points.
* A '''[[Field goal (gridiron football)|field goal]]''' (FG) is worth 3 points, and it is scored by kicking the ball over the crossbar and through the goal posts (uprights). Field goals may be placekicked (kicked when the ball is held vertically against the ground by a teammate) or [[drop kick|drop-kicked]] (extremely uncommon in the modern game, with only two successes in the last 60 years). A field goal is usually attempted on fourth down instead of a punt when the ball is close to the opponent's goal line, or, when there is little or no time left to otherwise score.
* A '''[[Safety (football score)|safety]]''', worth 2 points, is scored by the opposing team when the team in possession at the end of a down is responsible for the ball becoming dead behind its own goal line. For instance, a safety is scored by the "defense" if a player of the offense is tackled, goes out of bounds, or fumbles the ball out of bounds in his own end zone. Safeties are relatively rare. Note that, though even more rare, the team initially on "offense" during a down can score a safety if a player of the original defense gains possession of the ball in front of his own goal line and then carries the ball or fumbles it into his own end zone where it becomes dead. However, if the ball becomes dead behind the goal line of the team in possession and its "opponent" is responsible for the ball being there (for instance, if the defense intercepts a forward pass in its own end zone and the ball becomes dead before the ball is advanced out of the end zone) it is a touchback: no points are scored and the team last in possession keeps possession with a first down at its own 20 yard line. In college, in the extremely rare instance that a safety is scored on a try, it is worth only 1 point.

===Kickoffs and free kicks===
Each half begins with a [[Kickoff (American football)|kickoff]]. Teams also kick off after scoring touchdowns and field goals. The ball is kicked using a kicking tee from the team's own {{convert|30|yd|m|sing=on}} line in the NFL and college football (as of the 2007 season). The other team's kick returner tries to catch the ball and advance it as far as possible. Where he is stopped is the point where the offense will begin its '''drive''', or series of offensive plays. If the kick returner catches the ball in his own end zone, he can either run with the ball, or elect for a '''[[touchback]]''' by kneeling in the end zone, in which case the receiving team then starts its offensive drive from its own 20 yard line. A [[touchback]] also occurs when the kick goes out-of-bounds in the end zone. A kickoff that goes out-of-bounds anywhere other than the end zone before being touched by the receiving team is a foul, and the ball will be placed where it went out of bounds or {{convert|30|yd|m}} from the kickoff spot, depending on which is more advantageous to the opposite team. Unlike with punts, once a kickoff goes 10 yards and the ball has hit the ground, it can be recovered by the kicking team. A team, especially one who is losing, can try to take advantage of this by attempting an [[onside kick]]. Punts and turnovers in the end zone can also end in a [[touchback]].

After safeties, the team that gave up the points must [[Safety (football score)#Free kick|free kick]] the ball to the other team from its own 20 yard line.

===Penalties===
{{see|American football rules}}
Fouls (a type of rule violation) are punished with '''penalties''' against the offending team. Most penalties result in moving the football towards the offending team's end zone. If the penalty would move the ball more than half the distance towards the offender's end zone, the penalty becomes half the distance to the goal instead of its normal value.

Most penalties result in replaying the down. Some defensive penalties give the offense an automatic first down. Conversely, some offensive penalties result in loss of a down (loss of the right to repeat the down). If a penalty gives the offensive team enough yardage to gain a first down, they get a first down, as usual.

If a foul occurs during a down, an official throws a yellow flag near the spot of the foul. When the down ends, the team that did not commit the foul has the option of accepting the penalty, or declining the penalty and accepting the result of the down.

A few of the most-common fouls include:
*'''False start:''' An offensive player illegally moves after lining up for--but prior to--the snap. Since the ball is dead, the down is not allowed to begin.
*'''Offside:''' A defensive or offensive player is on the wrong side of the ball when the ball is snapped. This foul occurs simultaneously with the snap.
*'''Holding:''' Illegally grasping or pulling an opponent other than the runner.
*'''[[Pass interference]]:''' Illegally contacting an opponent to prevent him from catching a forward pass.
*'''Delay of game:''' Failing to begin a new play after a certain time from the end of the last one.
*'''Face mask:''' Grasping the face mask of another player while attempting to block or tackle him.
*'''Illegal block in the back:''' A blocker contacting a member of the opposing team in the back and above the waist.
*'''Clipping''': A blocker contacting an opponent from behind and at or below the waist.
*'''Chop Block''': Occurs when an offense player tries to cut block a defensive player that is already being blocked by another offensive player.

===Variations===
Variations on these basic rules exist, particularly [[Touch football (American)|touch]] and [[flag football]], which are designed as non-contact or limited-contact alternatives to the relative [[violence]] of regular American football. In touch and flag football, tackling is not permitted. Offensive players are "tackled" when a defender tags them or removes a flag from their body, respectively. Both of these varieties are played mainly in informal settings such as [[Intramural sports|intramural]] or youth games. Another variation is "wrap", where a player is "tackled" when another player wraps his arms around the ball carrier. Professional, intercollegiate, and varsity-level high school football invariably use the standard tackling rules.

Another variation is with the number of players on the field. In sparsely populated areas, it is not uncommon to find high school football teams playing [[nine-man football]], [[eight-man football]] or [[six-man football]]. Players often play on offense as well as defense. The [[Arena Football League]] is a league that plays eight-man football, but also plays indoors and on a much smaller playing surface with rule changes to encourage a much more offensive game.

Another variation often played by American children is called Catch and Run. In this game, the children split into two teams and line up at opposite sides of the playing field. One side throws the ball to the other side. If the opposing team catches the ball, that player tries to run to the throwing teams touchdown without being tagged/tackled. If no one catches the ball or if the player is tagged/tackled, then that team has to throw the ball to the opposing team. This repeats until the game (or recess period) is deemed over.

==Players==
{{Main|American football positions}}
{{seealso|Formation (American football)}}

[[Image:American football positions.svg|frame|right|This diagram shows typical offensive and defensive formations. The offense (blue) consists of the quarterback (QB), fullback (FB), tailback (TB), wide receivers (WR), tight end (TE), and offensive linemen (C, OG, OT). The defense (red) consists of the defensive line (DL, DE), linebackers (LBs), cornerbacks (CB), strong safety (SS) and free safety (FS). Because teams can change any or all of the players between plays, the number of players at certain positions may differ on a given play. Here the offense is in the Normal I-Formation while the defense is in a 4-3 Normal.]]

Most football players have highly specialized roles. At the college and NFL levels, most play only offense or only defense.

===Offense===
* The '''[[offensive line]]''' (OL) consists of five players whose job is to protect the passer and clear the way for runners by blocking members of the defense. The lineman in the middle is the Center. Outside the Center are the Guards, and outside them are the Tackles. Except for the center, who snaps the ball to one of the backs, offensive linemen generally do not handle the ball.
* The '''[[quarterback]]''' (QB) receives the snap from the center on most plays. He then hands or tosses it to a running back, throws it to a receiver or runs with it himself. The quarterback is the leader of the offense and calls the plays that are signaled to him from the sidelines.
*'''[[Running back]]s''' (RB) line up behind or beside the QB and specialize in running with the ball. They also block, catch passes and, on rare occasions, pass the ball to others or even receive the snap. If a team has two running backs in the game, usually one will be a halfback (HB) or tailback (TB), who is more likely to run with the ball, and the other will usually be a fullback (FB), who is more likely to block.
*'''[[Wide receiver]]s''' (WR) line up near the sidelines. They specialize in catching passes, though they also block for running plays or downfield after another receiver makes a catch.
*'''[[Tight end]]s''' (TE) line up outside the offensive line. They can either play like wide receivers (catch passes) or like offensive linemen (protect the QB or create spaces for runners). Sometimes an offensive lineman takes the tight end position and is referred to as a '''tackle eligible'''.<ref>[http://www.profootballhof.com/hof/member.jsp?player_id=158 Member - Pro Football Hall of Fame<!-- Bot generated title -->]</ref>

At least seven players must line up on the line of scrimmage on every offensive play. The other players may line up anywhere behind the line. The exact number of running backs, wide receivers and tight ends may differ on any given play. For example, if the team needs only 1 yard, it may use three tight ends, two running backs and no wide receivers. On the other hand, if it needs 20 yards, it may replace all of its running backs and tight ends with wide receivers.

===Defense===
In contrast to members of the offense, the rules of professional football ([http://www.nfl.com/rulebook NFL Rulebook]) and American college football [http://www.ncaa.org/library/rules/2007/2007_football_rules.pdf NCAA Rulebook]) do not specify starting position, movement, or coverage zones for members of the defensive team, except that they must be in the defensive zone at the start of play. The positions, movements and responsibilities of all defensive players are assigned by the team by selection of certain coverages, or patterns of placement and assignment of responsibilities. The positional roles are customary. These roles have varied over the history of American football. The following are customary defensive positions used in many coverages in modern American football.
* The '''[[defensive line]]''' consists of three to six players who line up immediately across from the offensive line. They try to occupy the offensive linemen in order to free up the linebackers, disrupt the backfield(behind the offensive line)of the offense, and [[Tackle (football move)#American and Canadian football|tackle]] the running back if he has the ball before he can gain yardage or the quarterback before he can throw or pass the ball. They are the first line of defense.
* Behind the defensive line are the '''[[linebacker]]s'''. They line up between the defensive line and defensive backs and may either rush the quarterback or cover potential receivers. Their main job is to cover the run up the middle.
* The last line of defense is known as the secondary, comprising at least three players who line up as '''[[defensive back]]s''', which are either [[cornerback]]s or safeties. They cover the receivers and try to stop pass completions. They occasionally rush the quarterback. However, this leaves the field wide open for passing.

===Special teams===
The units of players who handle kicking plays are known as [[special teams]]. Three important special-teams players are the [[punter (football position)|punter]], who handles punts, the [[placekicker]] or kicker, who kicks off and attempts field goals and extra points, and the [[long snapper]], who snaps the ball for extra points, field goals, and punts. Also included on special teams are the returners. These players return punts or kickoffs and try to get in good field position. These players can also score touchdowns.

===Uniform numbering===
[[Image:Dennis Rogers.jpg|thumb|right|Quarterback [[2007 Armed Forces Bowl|Shaun Carney]] has uniform number 5.]]
In the NFL, ranges of [[Uniform number (American football)|uniform numbers]] are (usually) reserved for certain positions:
*1–9: Quarterbacks, kickers, and punters
*10–19: Quarterbacks, kickers, punters, and wide receivers
*20–49: Running backs and defensive backs (tight ends are also permitted to wear numbers in the range 40-49 if no numbers are available in the range 80-89)
*50–59: Centers and linebackers
*60–79: Offensive and defensive linemen
*80–89: Wide receivers and tight ends
*90–99: Defensive linemen and linebackers

NCAA and high school rules specify only that offensive linemen must have numbers in the 50-79 range, but the NCAA "strongly recommends" that quarterbacks and running backs have numbers below 50 and wide receivers numbers above 79. This helps officials as it means that numbers 50 to 79 are [[eligible receiver|ineligible receivers]], or players that may never receive a forward pass. There are no numbering restrictions on defensive players in the NCAA, other than that no two players with the same jersey number can be on the field at the same time.

==Basic strategy==
{{Main|American football strategy}}

Because the game stops after every down, giving teams a chance to call a new play, strategy plays a major role in football. Each team has a '''playbook''' of dozens to hundreds of plays. Ideally, each play is a scripted, strategically sound team-coordinated endeavor. Some plays are very safe; they are likely to get only a few yards. Other plays have the potential for long gains but at a greater risk of a loss of yardage or a turnover.

Generally speaking, rushing plays are less risky than passing plays. However, there are relatively safe passing plays and risky running plays. To deceive the other team, some passing plays are designed to resemble running plays and vice versa. These are referred to as play-action passes and draws. There are many trick or gadget plays, such as when a team lines up as if it intends to punt and then tries to run or pass for a first down. Such high-risk plays are a great thrill to the fans when they work. However, they can spell disaster if the opposing team realizes the deception and acts accordingly.

The defense also plans plays in response to expectations of what the offense will do. For example, a "blitz" (using linebackers or defensive backs to charge the quarterback) is often attempted when the team on defense expects a pass. A blitz makes downfield passing more difficult but exposes the defense to big gains if the offensive line stems the rush.

Many hours of preparation and strategizing, including film review by both players and coaches, go into the days between football games. This, along with the demanding physicality of football (see below), is why teams typically play at most one game per week.

==Physicality==
{{main|Health issues in American football|Football protective equipment}}
[[Image:AmericanFootballTraining.jpg|thumb|right|A halfback leads fellow backs through an agility drill at the Air Force Academy]]

American football is a collision sport. To stop the offense from advancing the ball, the defense must tackle the player with the ball by knocking or pulling him down. As such, defensive players must use some form of physical contact to bring the ball-carrier to the ground, within certain rules and guidelines. Tacklers cannot kick or punch the runner. They also cannot grab the face mask of the runner's helmet or lead into a tackle with their own helmet. Despite these and other rules regarding unnecessary roughness, most other forms of tackling are legal. Blockers and defenders trying to evade them also have wide leeway in trying to force their opponents out of the way. Quarterbacks are regularly hit by defenders coming on full speed from outside the quarterback's field of vision. This is commonly known as a blindside.

To compensate for this, players must wear special protective equipment, such as a padded plastic [[football helmet|helmet]], [[shoulder pads]], hip pads and knee pads. These protective pads were introduced decades ago and have improved ever since to help minimize lasting [[injury]] to players. An [[unintended consequence]] of all the safety equipment has resulted in increasing levels of violence in the game. Players may now hurl themselves at one another at high speeds without a significant chance of injury. The injuries that do result tend to be severe and often season or career-ending and sometimes fatal. In previous years with less padding, tackling more closely resembled tackles in [[Rugby football]]. Better helmets have allowed players to use their helmets as weapons. This form of tackling is particularly unwise, because of the great potential for brain or spinal injury. All this has caused the various leagues, especially the NFL, to implement a complicated series of penalties for various types of contact. Most recently, virtually any contact with the helmet of a defensive player on the quarterback, or any contact to the quarterback's head, is now a foul.

Despite protective equipment and rule changes to emphasize safety, injuries remain very common in football. It is increasingly rare, for example, for NFL quarterbacks or running backs (who take the most direct hits) to make it through an entire season without missing some time to injury. Additionally, 28 football players died from direct football injuries in the years 2000-05 and an additional 68 died indirectly from dehydration or other examples of "non-physical" dangers, according to the National Center for Catastrophic Sport Injury Research.<ref>[http://www.unc.edu/depts/nccsi/FootballInjuryData.htm Annual Survey of Football Injury Research 1931 - 2005], [http://www.unc.edu/depts/nccsi/ National Center for Catastrophic Sport Injury Research]. Updated January 18, 2006. Accessed October 31, 2006 </ref> [[Concussion]]s are common, with about 41,000 suffered every year among high school players according to the Brain Injury Association of Arizona.<ref>[http://www.prnewswire.com/cgi-bin/stories.pl?ACCT=104&STORY=/www/story/08-23-2005/0004093186&EDATE= Studies Suggest 10% of Arizona High School Football Players Will Suffer a Concussion During This Coming Season] [[PR Newswire]] press release from the [http://www.biaaz.org Brain Injury Association of Arizona], August 23, 2005. Accessed October 31, 2006</ref> In 1981, former U.S. President [[Ronald Reagan]], who played football in high school, commented on the contact of the sport: "[Football] is the last thing left in civilization where men can literally fling themselves bodily at one another in combat and not be at war."<ref name="Reagan">{{cite book|last = D'Souza|first=Dinesh|title=Ronald Reagan: How an Ordinary Man Became an Extraordinary Leader|publisher=Free Press|date=February 23, 1999|pages=40|isbn=0684848236}}</ref>

Extra and optional equipment such as neck rolls, spider pads, rib protectors (referred to as "flak jackets"), and [[elbow pads]] help against injury as well, though they do not tend to be used by the majority of players due to their lack of requirement.

The danger of football and the equipment required to reduce it make regulation football impractical for casual play. [[Flag football]] and [[touch football (American)|touch football]] are less violent variants of the game popular among recreational players.

==Organization in the United States==
{{main|American football in the United States}}

===Calendar===
Traditionally, football is an autumn sport. A season typically begins in mid-to-late August and runs through December, into January. The professional playoffs run through January, and the [[Super Bowl]] is often played in the first week of February.
[[Image:TurkeyBowl.jpg|thumb|left|250px|Amateurs [[American football on Thanksgiving|playing on Thanksgiving]]]]
The [[NFL draft]] is usually held in April, in which eligible college football players are selected by NFL teams, the order of selection determined by the teams' final regular season records.

It is a long-standing tradition in the United States (though not universally observed) that high school football games are played on Friday night, college games on Saturday, and professional games on Sunday.

In the 1970s, the NFL began to schedule one game on [[Monday night football|Monday nights]]. Beginning in 2006, the NFL began scheduling games on Thursday and Saturday nights after the college football regular season concludes in mid-November, aired on the [[NFL Network]].

In recent years, nationally televised Thursday night college games have become a weekly fixture on [[ESPN]], and most nights of the week feature at least one college game, though most games are still played on the traditional Saturday.

Certain fall and winter [[holiday]]s&mdash;such as the NFL's [[Thanksgiving Classic]] and numerous [[New Year's Day]] college [[bowl games]]&mdash;have traditional football games associated with them.

Despite this, there are a few professional leagues that have played in the spring, mainly to avoid competition with the established leagues. Examples include the now defunct [[XFL]], the [[United States Football League]], and the proposed [[All American Football League]]. [[Indoor football]] is played primarily in spring for this same reason.

==Outside the United States==

Outside the United States, the sport is referred to as "American football" (or a translation thereof) to differentiate it from other [[football]] codes such as [[association football]] (soccer) and [[rugby football]]. In [[Australia]] and [[New Zealand]] the game is known as [[gridiron football]], although in the United States the term ''gridiron'' refers only to the playing field itself.<ref>"gridiron." The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. 01 Oct. 2007. [http://dictionary.reference.com/browse/gridiron].</ref> In much of the world, the term [[Football (word)|football]] is unambiguous and refers to association football.

The NFL has attempted to introduce the game to other nations and operated a developmental league, [[NFL Europa]]<!--note 2006 name change from NFL Europe-->, with teams in five German cities and one in the Netherlands, but this league folded following the 2007 season. The professional [[Canadian Football League]] and collegiate [[Canadian Interuniversity Sport]] play under the only slightly different [[Canadian football|Canadian rules]].

In the UK, 46 teams play in the [[British American Football League]]. The BAFL is comprised of three levels: The Premier, of which there are 7 teams; BAFL 1, of which there are 12 teams; and BAFL 2, of which there are 27 teams. While the lower level teams have their own championship games during BritBowl Weekend, only Premier league teams face each other in the [[BritBowl]] which is it traditionally held in [[Sheffield]]'s [[Don Valley Stadium]]. Unlike the NFL, the BAFL season is played through the summer (April to September), with the British university season spanning the autumn and winter.

In [[Japan]], the [[X-League]] is a professional league with 60 teams in four divisions, using [[promotion and relegation]]. After the post-season playoffs, the X-League champion is determined in the Japan X Bowl. There are also over 200 universities fielding teams, with the national collegiate championship determined by the [[Koshien Bowl]]. The professional and collegiate champions then face each other in the [[Rice Bowl]] to determine the national champion.

In Germany, the [[German Football League]] whose elite division is called [[bundesliga (disambiguation)|bundesliga]], has 12 teams partitioned into north and south conferences. The finalists from the playoffs determine the German champion during the [[German Bowl]].

In Hungary, 18 registered teams participate in a the [http://www.sportmedia.hu/amerikaifoci/english/ MAFL]'s two division league structure. The sport has grown significantly since 2004 and with some top Division I teams participating in the [[Central European Football League|CEFL]].

In Norway, div I consists of only two teams, [[Oslo Vikings]] and [[Eidsvoll 1814's]]. These two teams also compete in the [[European Football League]] but they play an annual game for the Norwegian Championship title. Norway has seven other teams that play in div II and this division is looked upon as the [[Norwegian Football League]].
The [[International Federation of American Football]] is the governing body for American football with 45 member associations from North and South America, Europe, Asia and Oceania. The IFAF also oversees the [[American Football World Cup]], which is held every four years. Japan won the first two World Cups, held in 1999 and 2003. Team USA, which had not participated in the previous World Cups, won the title in 2007.

Major American leagues have also held some regular season games outside of the United States. On October 2, 2005, the Arizona Cardinals and San Francisco 49ers played the first regular season NFL game outside of the United States, in Mexico City's Estadio Azteca,<ref>[http://www.bizjournals.com/sanfrancisco/stories/2005/03/14/daily27.html S.F. 49ers, Arizona Cardinals to kick off in Mexico - San Francisco Business Times:<!-- Bot generated title -->]</ref> From 2007, the NFL has played or has plans to play at least one regular season game outside of the United States. The [[National Collegiate Athletic Association|NCAA]] will also play games outside of the U.S. In 2012, The United States Naval Academy will play the University of Notre Dame in [[Dublin, Ireland]].[http://und.cstv.com/sports/m-footbl/spec-rel/111005aac.html]

==See also==
{{American football portal}}
*[[American football in the Netherlands]]
*[[American football on Thanksgiving]]
*[[Comparison of American football and rugby league]]
*[[Eight-man football]]
*[[Fantasy football (American)]]
*[[Glossary of American football]]
*[[Gridiron football]]
*[[List of American football players]]
*[[List of American football stadiums by capacity]]
*[[List of American football teams in Germany]]
*[[List of American football teams in the Netherlands]]
*[[List of American football teams in the United Kingdom]]
*[[List of defunct sports leagues]]
*[[List of leagues of American and Canadian football]]
*[[Nine-man football]]
*[[Pro Football Hall of Fame]]
*[[Six-man football]]
*[[Sprint Football]]
*[[Strat-O-Matic Football]]
*[[Street football (American)]]

==Notes==
{{reflist}}

==References==
*{{cite web|url = http://www.nfl.com/rulebook/digestofrules|title = Digest of Rules|publisher = National Football League|accessdate = 2007-10-31}}
*{{cite web|url = http://www.nfl.com/history|title = History and the basics|publisher = National Football League|accessdate = 2005-12-28}}
*{{cite web|url = http://www.thesportjournal.org/2005Journal/Vol8-No4/starkey.asp|title = Playing with the Percentages When Trailing by Two Touchdowns|publisher = Montana State University|accessdate = 2005-12-24}}

==Further reading==
*''[[Sports Illustrated]]'' magazine dated December 4, 2005; "Football America", a series of articles attesting to the pervasive popularity of American football in the United States at all levels.

==External links==
{{Wiktionary}}
{{Commons|American football}}

*[http://www.ncaa.org/library/rules/2006/2006_football_rules.pdf NCAA's complete college football rules; available as a PDF file]
*[http://memory.loc.gov/cgi-bin/query/S?ammem/papr:@FILREQ(@field(TITLE+@od1(Chicago-Michigan+football+game++))+@FIELD(COLLID+workleis)) Movie of 1903 football game between the University of Chicago and the University of Michigan]
*National Football League [http://www.nfl.com/rulebook/signals Official Signals].
*[http://www.unc.edu/depts/nccsi/SurveyofFootballInjuries.htm Annual Survey of Football Injury Research]
*[http://news.bbc.co.uk/sport1/hi/other_sports/american_football/3192002.stm Brief explanation of the sport by the BBC aimed at a non-american audience]
*{{dmoz|Sports/Football/American/|American Football}}
*[http://www.americanyouthfootball.com American Youth Football]
*[http://www.football-rules.com Football rules and the history]

{{Hybridsports}}
{{Team Sport}}

[[Category:American football| ]]

{{Link FA|pl}}
{{Link FA|pt}}
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