Halfback (American football)
A halfback, sometimes referred to as a tailback, is an offensive position in American football, which lines up in the backfield and generally is responsible for carrying the ball on running plays. Historically, from the 1870s through the 1950s, the halfback position was both an offensive and defensive position. In the related sport of Canadian football, halfback is a defensive rather than offensive position.
Early offensive backfields 
Before the emergence of the T-formation in the 1940s, all members of the offensive backfield were legitimate threats to run or pass the ball. Most teams used four offensive backs on every play: a quarterback, two halfbacks, and a fullback. The quarterback began each play a quarter of the way back, the halfbacks began each play side by side and halfway back, and the fullback began each play the farthest back.
The sport's first triple threat, Bradbury Robinson of St. Louis University, ran, passed, received and punted out of the halfback position. It was as a halfback that Robinson threw the first legal forward pass to teammate Jack Schneider in a game at Carroll College on September 5, 1906.
Now that most offensive formations have only one or two running backs, the original designations do not mean as much, as the fullback is now usually a lead blocker (technically a halfback), while the halfback or tailback (called such because he stands at the "tail" of the I) lines up behind the fullback.
Responsibilities of the halfback 
The halfback position is one of the more glamorous positions on the field, and is commonly viewed as a requirement for a team's success. They are responsible for carrying the ball on the majority of running plays, and may frequently be used as a receiver on short passing plays. Occasionally, they line up as additional wide receivers. When not serving either of these functions, the primary responsibility of a halfback is to aid the offensive linemen in blocking, either to protect the quarterback or another player carrying the football. Sometimes the halfback or tailback can catch the ball from the backfield as he is an eligible receiver.
Running ability 
No position in American football can perform his duties successfully without the help of other players. Like the wide receiver, who generally cannot make big plays without the quarterback passing to him (with the exception of the end-around play), the halfback needs good blocking from the offensive line and fullback to successfully gain yardage. Also, a halfback will generally have more rushing attempts than a receiver will have receptions. This is mainly because most football teams have one primary halfback to receive most of the carries, while successful passes will generally be spread between a number of different receivers (wide receivers, tight ends, running backs).
Characteristics of a halfback 
Physical characteristics 
There is a great diversity in those who play at the running back position. At one extreme are smaller, faster players. These fast, agile, and elusive running backs are often called "scat backs" because their low center of gravity and maneuverability allow them to dodge tacklers. Hall-of-famer Barry Sanders of the Detroit Lions exemplifies this running style. "Scat backs" still active in the NFL include Chris Johnson of the Tennessee Titans, Danny Woodhead of the New England Patriots, and Reggie Bush of the Detroit Lions.
At the other extreme are "power backs": bigger, stronger players who can break through tackles using brute strength and raw power. They are usually (but not always) slower runners compared to other backs, and typically run straight ahead (or "North-and-South" in football terminology) rather than dodging to the outside edges of the playing field (i.e. running "East-and-West") like shorter, quicker, lighter backs will often do. Historically, some of the greatest halfbacks were "power" running backs, such as John Riggins and Jerome Bettis. More recent examples are Marshawn Lynch of the Seattle Seahawks, Steven Jackson of the Atlanta Falcons, Alfred Morris of the Washington Redskins, Frank Gore of the San Francisco 49ers and Peyton Hillis of the Kansas City Chiefs.
Receiving ability 
In addition to skill at running the ball, some halfbacks in the National Football League are known for their skill at catching passes. The role of the halfback as a receiver out of the backfield has expanded greatly in the NFL over the years, and a versatile halfback who provides his team with running and pass-catching abilities is highly valued. On passing plays, a halfback will often run a safe route, such as a hook, creating a safe target for a quarterback to throw to if all other receivers are covered. The increase in demand for halfbacks with good receiving abilities can be attributed to the rise in popularity of the West Coast offense and its variants, which often requires its halfbacks to catch passes on a regular basis. A great early example of a system that combined accomplished rushing skills with receiving ability is the offense of the San Francisco 49ers of the 1980s and 1990s under Bill Walsh and George Seifert. Their teams featured two Pro Bowl running backs who also had excellent receiving skills in Roger Craig and Ricky Watters. Craig became the first player in NFL history to both rush and receive more than 1,000 yards in a season. Currently Marshall Faulk is one of the top 20 pass catchers in NFL history. A good example of a dual threat running and pass-catching halfback is LaDainian Tomlinson; in 2003, while with the San Diego Chargers, Tomlinson rushed for 1,645 yards and caught 100 passes for 725 yards, giving him 2,370 total yards from the line of scrimmage, and he became the first NFL player ever to rush for over 1,000 yards and catch 100 passes in a season. Some teams have a halfback who is more skilled at catching short passes than the starting halfback on the team, and/or is better at pass blocking or "picking up the blitz" than that of the other backs. Known as a "third down" back, he is often put in the game in third down and long situations where a pass is needed to pick up a first down. He can also be used to fool the defense by making them think he is being put into the game for a pass play, when the play is actually a run. Mike Tolbert of the Carolina Panthers is a good current example of this kind of back.
Halfbacks are also required to help the offensive line in passing situations, and, in the case of the fullback, running plays. Halfbacks will often block blitzing linebackers or safeties on passing plays when the offensive line is occupied with the defensive linemen. On running plays, the fullback will often attempt to tear a hole in the defensive line for the halfback to run through. Effective blocking backs are usually key components for a running back's success (as seen in LaDainian Tomlinson record-breaking season in 2006).
Goal line backs 
Many teams also have a halfback designated as a "goal line back" or "short yardage specialist". This halfback comes into the game in short yardage situations when the offense needs only one to five yards to get a first down. They also come into the game when the offense nears the goal-line. Normally when an offense gets inside the 5 yard line they send in their goal-line formation which usually includes eight blockers, a quarterback, a halfback, and a fullback. The closer they are to the goal-line the more likely they are to use this formation. If a certain halfback is used often near the goal-line he is called the goal-line back. Short yardage and goal-line backs are usually larger power backs that are not prone to fumbling. Their job is to get the first down or touchdown by muscling through or pushing a large mass of players that are being blocked without dropping the ball. Players like John Kuhn, Thomas Jones, and Brandon Jacobs specialize in short-yardage situations, like third and short.
Contributing to special teams 
A halfback might be called upon to return punts and kickoffs on special teams. Although this is most often done by wide receivers and defensive backs, such as cornerbacks (because they are generally the fastest players on the team), some halfbacks have enough speed and talent to perform this role. The NFL's current all time leading in kickoff return yards (14,014 yards) and punt return yards (4,999) by a halfback is Brian Mitchell. He also gained 1,967 rushing yards, 2,336 receiving yards, and 15 fumble return yards, giving him a total of 23,330 all-purpose yards, the second most in NFL history behind Jerry Rice.
Passing ability 
On rare occasions, a halfback is asked to throw the ball when running a HB option. This play is generally referred to as a half-back pass, regardless if the player throwing the football is a tailback or fullback. This play is risky because most halfbacks are inexperienced passers, and so it is often run only by certain halfbacks more skilled at passing than most. The halfback can also throw the ball while running a direct snap play where the center snaps the ball to halfback directly. This has become particular in teams that use the Wildcat formation, most prominently the Miami Dolphins, where running back Ronnie Brown would run, pass, and receive out of this set.
See also 
|Positions in American football and Canadian football|
|Linemen||Guard, Tackle, Center||Linemen||Tackle, End, Nose tackle||Kicking players||Placekicker, Punter, Kickoff specialist|
|Quarterback||Linebackers||Snapping||Long snapper, Holder|
|Backs||Halfback (Tailback), Fullback, H-back||Backs||Cornerback, Safety, Halfback||Returning||Punt returner, Kick returner|
|Receivers||Wide receiver, Tight end, Slotback||Nickelback, Dimeback||Tackling||Gunner|
|Formations (List) — Nomenclature|