Offside is one of the laws of association football, codified in Law 11 of the Laws of the Game. The law states that a player is in an offside position if any of their body except the hands and arms is in the opponents' half of the pitch and closer to the opponents' goal line than both the ball and the second-last opponent (usually, but not necessarily always, the last defensive player in front of the goalkeeper). Being in an offside position is not an offence in itself; a player who was in an offside position at the moment the ball last touched, or was played by a team mate, must then become involved in active 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 place where the offending player became involved in active play.
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. Assistant referees communicate that an offside offence has occurred by raising a signal flag. :191 However, as with all officiating decisions in the game, adjudicating offside is ultimately up to the referee, who can overrule the advice of their assistants if they see fit.
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 the opponents' goal line than both the ball and the second-last opponent." The 2005 edition of the Laws of the Game included a new IFAB decision that stated, "In the definition of offside position, 'nearer to his opponents' goal line' means 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. The arms are not included in this definition". By 2017, the wording had changed to say that, in judging offside position, "The hands and arms of all players, including the goalkeepers, are not considered."  In other words, a player is in an offside position if two conditions are met:
- Any part of the player's head, body or feet is in the opponents' half of the field (excluding the half-way line).
- Any part of the player's head, body or feet is closer to the opponents' goal line than both the ball and the second-last opponent. 
(The goalkeeper counts as an opponent in the second condition, but it is not necessary that the last opponent be the goalkeeper.)
A player in an offside position at the moment the ball is touched or played by a team mate is only penalised for committing an offside offence if, in the opinion of the referee, he becomes involved in active play by:
- Interfering with play
- "playing or touching the ball passed or touched by a team-mate"
- 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 or
- clearly attempting to play a ball which is close to him when this action impacts on an opponent or
- making an obvious action which clearly impacts on the ability of an opponent to play the ball"
- Gaining an advantage by playing the ball or interfering with an opponent when it has
- "- rebounded or been deflected off the goalpost, crossbar, match official or an opponent
- - been deliberately saved by any opponent" 
In addition to the above criteria, in the 2017-18 edition of the Laws of the Game, the IFAB made a further clarification that, "In situations where a player moving from, or standing in, an offside position is in the way of an opponent and interferes with the movement of the opponent towards the ball this is an offside offence if it impacts on the ability of the opponent to play or challenge for the ball." 
There is no offside offence if a player receives the ball directly from a goal kick, a corner kick, or a throw-in. It is also not an offence if the ball was last deliberately played by an opponent (except for a deliberate save). In this context, according to the IFAB, "A ‘save’ is when a player stops, or attempts to stop, a ball which is going into or very close to the goal with any part of the body except the hands/arms (unless the goalkeeper within the penalty area)." 
Since offside is judged at the time the ball is touched or played by a teammate, not when the player receives the ball, it is possible for a player to receive the ball significantly past the second-to-last opponent, or even the last opponent, without committing an offence. This used to be expressed in the Law by International Board Decision 1 to Law 11 using the following phrase, "A player who is not in an off-side position when one of his colleagues passes the ball to him or takes a free-kick, does not therefore become off-side if he goes forward during the flight of the ball." 
Determining whether a player is "involved in active play" can be complex. The quote, "If he's not interfering with play, what's he doing on the pitch?" has been attributed to Bill Nicholson and Danny Blanchflower. In an effort to avoid such criticisms, which were based on the fact that phrases such as "interfering with play", "interfering with an opponent" and "gaining an advantage" were not clearly defined, FIFA issued new guidelines for interpreting the offside law in 2003 and these were incorporated into Law 11 in July 2005. The new wording sought to define the three cases more precisely, but a number of football associations and confederations continued to request more information about what movements a player in an offside position could make without interfering with an opponent. In response to these requests IFAB circular 3 was issued in 2015 to provide additional guidance on the criteria for interfering with an opponent. This additional guidance is now included in the main body of the law and forms the last 3 conditions under the heading "Interfering with an opponent" as shown above. The circular also contained additional guidance on the meaning of a save, in the context of a ball that has "been deliberately saved by any opponent." 
The sanction for an offside offence is an indirect free kick for the opponent at the place where the player becomes involved in active play, even if it is in the player’s own half of the field of play.
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.:176 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 partly lowering their flag to an angle that signifies the location of the offence::192
- Flag pointed at a 45-degree angle downwards: offence has occurred in the third of the pitch nearest to the assistant referee;:73
- Flag parallel to the ground: offence has occurred in the middle third of the pitch;:73
- Flag pointed at a 45-degree angle upwards: offence has occurred in the third of the pitch furthest from the assistant referee.:73
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. This was similar to the current offside law in rugby, under which any player between the ball and the opponent's goal who takes part in play, is liable to be penalised.  By contrast, the original Sheffield Rules had no offside rule, and players known as "kick-throughs" were positioned permanently near the opponents' goal.
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.
6. When a player has kicked the ball any one of the same side who is nearer to the opponent's goal line is out of play and may not touch the ball himself, nor in any way whatever prevent any other player from doing so until the ball has been played, but no player is out of play when the ball is kicked from behind the goal line
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.
In the 1987–88 season, the Football Association was authorised by the International Football Association Board to test an experimental rule change, whereby no attacker could be offside directly from a free-kick.  
Pioneered in the early 20th century by Arsenal 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. Now that changes to the interpretations of "interfering with play, interfering with an opponent and gaining an advantage" mean a player is not guilty of an offside offence unless he becomes directly and clearly involved in active play, players not involved in active play cannot be "caught offside", making the tactic more risky.
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- 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 2017-18 — Law 11 - Offside is on pages 93-95. Pages 195 - 201 give visual diagrams of situations in which offside offences occur or do not occur under Law 11.
- FIFA Offside Presentation, June 2005
- Offside explained at AskTheRef.com
- FIFA interactive guide
- Professional Referee Organization offside discussion, from 2015 pre-season (includes video examples)