Offside (association football)
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Offside is one of the rules of association football, codified in Law 11 of the Laws of the Game. The law states that players in an offside position, when the ball is touched or played by a teammate, may not become actively involved in the play. A player is in an offside position when he or she is in the opponents' half of the pitch and closer to the opponents' goal line than both the ball and the second-to-last opponent (usually the last defensive player in front of the goalkeeper). Being in an offside position is not an offence in itself; at the moment the ball touches or is played by the player's team, the player must also be "actively involved in the play" in the opinion of the referee, in order for an offence to occur. When the offside offence occurs, the referee stops play and awards an indirect free kick to the defending team from the position of the offending player.
The offside offence is neither a foul nor a misconduct, players are never booked or sent off for offside. Like fouls however,any play (such as the scoring of a goal) that occurs after an offence occurs but before the referee is able to stop play is nullified.
One of the main duties of the assistant referees is to assist the referee in adjudicating offside — their position on the sidelines giving a more useful view sideways across the pitch.:86, 95 Assistant referees communicate that an offside offence has occurred by raising a signal flag.:97
The application of the offside rule may be considered in three steps: offside position, offside offence and offside sanction.
A player is in an offside position if he is in the opposing team's half of the field and is also "nearer to his opponents' goal line than both the ball and the second-last opponent."  being "nearer to an opponent's goal line" meant that "any part of his head, body or feet is nearer to his opponents' goal line than both the ball and the second-last opponent". By the 2014/2015 Laws, the interpretation included the proviso that "The arms are not included in this definition".:108
A player in an offside position at the moment the ball is touched or played by a team mate is only committing an offside offence if, in the opinion of the referee, he becomes actively involved in play by:
- Interfering with play
- "playing or touching the ball passed or touched by a team-mate":108
- Interfering with an opponent
- "preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s line of vision or challenging an opponent for the ball":108
- Gaining an advantage by being in an offside position
- "playing a ball that rebounds or is deflected to him off the goalpost, crossbar or an opponent having been in an offside position; or that rebounds, is deflected or is played to him from a deliberate save by an opponent having been in an offside position":108
Since offside is judged at the time the ball is touched or played by a teammate, not when the player receives the ball, citation needed][
Determining whether a player is in "active play" can be complex. The quote, "If you're not interfering with play, what are you doing on the pitch?" has been attributed to Brian Clough and Danny Blanchflower. 
The restart for an offside sanction is an indirect free kick for the opponent at the place where the off-sided player was at the time the teammate passed or touched the ball. This is defined as where the infringement took place.
In enforcing this rule, the referee depends greatly on an assistant referee, who generally keeps in line with the second-to-last opponent, the ball, or the halfway line, whichever is closer to the goal line of their relevant end.:86 An assistant referee signals that an offside offence has occurred by first raising their flag upright without movement and then, when acknowledged by the referee, by raising their flag in a manner that signifies the location of the offence:
- Flag pointed at a 45-degree angle downwards: offence has occurred in the third of the pitch nearest to the assistant referee;:97
- Flag parallel to the ground: offence has occurred in the middle third of the pitch;:97
- Flag pointed at a 45-degree angle upwards: offence has occurred in the third of the pitch furthest from the assistant referee.:97
Some researchers believe that offside officiating errors are "optically inevitable". It has been argued that human beings and technological media are incapable of accurately detecting an offside position quickly enough to make a timely decision. Sometimes it simply is not possible to keep all the relevant players in the visual field at once. There have been some proposals for automated enforcement of the offside rule.
Offside rules date back to codes of football developed at English public schools in the early nineteenth century. These offside rules, which varied widely between schools, were often much stricter than the modern game. In some of them, a player was "off his side" if he was standing in front of the ball. 
Offside was probably part of the "Cambridge Rules" from their inception in 1848. A ruleset dating from 1856 found in the library of Shrewsbury School is probably closely modelled on the Cambridge Rules and is thought to be the oldest set still in existence. Rule No. 9 required more than three defensive players to be ahead of an attacker who plays the ball. The rule states:
If the ball has passed a player and has come from the direction of his own goal, he may not touch it till the other side have kicked it, unless there are more than three of the other side before him. No player is allowed to loiter between the ball and the adversaries' goal.
As football developed in the 1860s and 1870s, the offside law proved the biggest argument between the clubs. Sheffield got rid of the "kick-throughs" by amending their laws so that one member of the defending side was required between a forward player and the opponents' goal. The compromise rule that was written into the Laws of the Game in 1866, and eventually adopted universally, was to adopt a form of the Cambridge rule, but with "at least three" rather than "more than three" opponents.
The rule changed to "two opponents" in 1925 and led to an immediate increase in goal-scoring. 4,700 goals were scored in 1,848 Football League games in 1924–25. This number rose to 6,373 goals (from the same number of games) in 1925–26.
In 1990 the law was amended to adjudge an attacker as onside if level with the second-to-last opponent. This change was part of a general movement by the game's authorities to make the rules more conducive to attacking football and help the game to flow more freely.
During the 1973–74 and 1974–75 seasons, an experimental version of the offside rule was operated in the Scottish League Cup and Drybrough Cup competitions. The concept was that offside should only apply in the last 18 yards of play (i.e. inside or beside the penalty area). To signify this, the horizontal line of the penalty area was extended to the touchlines. FIFA President Sir Stanley Rous attended the 1973 Scottish League Cup Final, which was played using these rules. The manager of one of the teams involved, Celtic manager Jock Stein, complained that it was unfair to expect teams to play under one set of rules in one game and then a different set a few days before or later. The experiment was quietly dropped after the 1974–75 season, as no proposal for a further experiment or rule change was submitted for the Scottish Football Association board to consider.
Pioneered in the early 20th century by Arsenal FC and later adopted by influential Argentine coach Osvaldo Zubeldía, the offside trap is a defensive tactic designed to force the attacking team into an offside position. Just before an attacking player is played a through ball, the last defender(s) move up field, isolating the attacker into an offside position. The execution requires careful timing by the defence and is considered a risk, since running up field against the direction of attack may leave the goal exposed.
- "Law 11 — Offside" (PDF). Laws of the Game 2014/2015. Zurich: Fédération Internationale de Football Association. 2014-06-27. p. 35. Retrieved 12 June 2015.
- "Law 10 — The Method of Scoring" (PDF). Laws of the Game 2014/2015. Zurich: Fédération Internationale de Football Association. 2014-06-27. p. 34. Retrieved 12 June 2015.
A goal is scored ... provided that no infringement of the Laws of the Game has been committed previously by the team scoring the goal.
- "Law 6 — The Assistant Referees" (PDF). Laws of the Game 2014/2015. Zurich: Fédération Internationale de Football Association. 2014-06-27. p. 28. Retrieved 12 June 2015.
- "Interpretation of the Laws of the Game and Guidelines for Referees" (PDF). Laws of the Game 2014/2015. Zurich: Fédération Internationale de Football Association. 2014-06-27. Retrieved 12 June 2015.
- Peters, Jerrad (2012-12-28). "Offside interpretations continue to evolve, logic be damned". The Score Blog. Archived from the original on 2014-07-28. Retrieved 13 June 2015.
- John Motson (1994-05-09). Commentary: Brazil vs Netherlands, World Cup 1994 (YouTube). United States: FIFA / BBC. Retrieved 2014-05-26.
- Oudejans, Raôul R. D.; Verheijen, Raymond; Bakker, Frank C.; Gerrits, Jeroen C.; Steinbrückner, Marten; Beek, Peter J. (2000), "Errors in judging 'offside' in football", Nature 404 (6773): 33–33, doi:10.1038/35003639
- FB Maruenda (2009), An offside position in football cannot be detected in zero milliseconds, doi:10.1038/npre.2009.3835.1, hdl:10101/npre.2009.3835.1
- B Maruenda (2004), "Can the human eye detect an offside position during a football match?", BMJ (British Medical Journal) 329 (7480): 1470–2, doi:10.1136/bmj.329.7480.1470, PMC 535985, PMID 15604187 Correction: "Can the human eye detect an offside position during a football match?", BMJ 330 (7484), 2005: 188, doi:10.1136/bmj.330.7484.188
- S Iwase, H Saito (2002), Tracking soccer player using multiple views, Proceedings of the IAPR Workshop on Machine Vision, CiteSeerX: 10
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- Carosi, Julian (2006). "The History of Offside" (PDF). Retrieved 12 April 2015.
- "150 years of Association Football ~ How the Rules have changed". Retrieved 2013-04-25.
- Russell, Grant (1 April 2011). "How the Scottish FA tried to revolutionise the offside law". www.sport.stv.tv. STV. Retrieved 13 December 2013.
- Wilson, Jonathan (13 April 2010), The Question: Why is the modern offside law a work of genius?
- Intercontinental Cup 1968, archived from the original on 6 Nov 2012
|Wikimedia Commons has media related to Offside (association football).|
- Laws of the Game 2014/2015 — Page 35 is Law 11. Pages 108–116 give referees interpretations and visual diagrams of situations in which an offside offences occur or do not occur under Law 11.
- FIFA Offside Presentation, June 2005
- Offside explained at AskTheRef.com
- FIFA interactive guide
- The Offside Rule, For beginners