Offside is a law in association football which effectively limits how far forward attacking players may be when involved in play. Simply put, a player cannot gain an advantage by waiting for the ball near the opposing goal with fewer than two opponents between him and the goal.
A player is in an offside position if he is in his opponents' half of the field and is nearer to his opponents' goal line than the ball, and all but one (or all) of his opponents. A player level with the second to last opponent is not in an offside position.
In 2005 The International Football Association Board agreed a new Decision in Law 11 that being 'nearer to his 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. The arms are not included in this definition." This is taken to mean that any part of the attacking player named in this Decision 2 has to be past the part of the second last defender closest to his goal line (excluding the arms) and past the part of the ball closest to the defenders' goal line.
In general, what this means is that either the attacking team should ensure the opposing team has at least two players (of which the opposition's goalkeeper is included) in front of the furthest forward player of the attacking team, or all players of the attacking team should be behind the ball such that it remains closer to the goal line than any of the player of the attacking team. If the goalkeeper is ahead of the play, then the forward will have to be in line with or behind two defenders (unless the forward is in his own half).
There is currently some controversy over whether a defender who has left the field of play is counted as active for the purposes of determining whether or not an attacker is offside. The Laws of the Game simply say that the "player is in an offside position if he is nearer to his opponents’ goal line than both the ball and the second last opponent", which is not definitive with regards to players being behind the goal line. The US Soccer Federation Advice to referees part 11.11 states that "A defender who leaves the field during the course of play and does not immediately return must still be considered in determining where the second to last defender is for the purpose of judging which attackers are in an offside position. Such a defender is considered to be on the touch line or goal line closest to his or her off-field position. A defender who leaves the field with the referee's permission (and who thus requires the referee's permission to return) is not included in determining offside position."
A player in an offside position is only committing an offside offence if, in the opinion of the referee, he is involved in active play "at the moment the ball touches or is played by one of his team." A player is not committing an offside offence if the player receives the ball directly from a throw-in, goal kick or corner kick.
Therefore a player who runs from an onside position into an offside position after the ball was touched or played by a team-mate is not penalised because a team-mate is no longer touching the ball.
Conversely, a player who is in an offside position when the ball is touched or played by a team-mate may potentially commit an offside offence even if he runs back in to an on-side position before receiving the ball. This potential remains until another player touches or plays the ball and offside position is reevaluated, or the ball goes out of play, or an opponent makes a controlled play on the ball. A player formerly in offside position who benefits from an ill-advised but deliberate play by an opponent is not judged offside.
Determining whether a player is in "active play" can be complex. FIFA issued new guidelines for interpreting the offside law in 2003 and these were incorporated in law 11 in July 2005. The new wording seeks to more precisely define the three cases as follows:
In practice, a player in an offside position may be penalised before playing or touching the ball if, in the opinion of the referee, no other team-mate in an onside position has the opportunity to play the ball.
Controversy regarding offside decisions is normally caused by what movements a player in an offside position can make without being judged to be interfering with an opponent. The famous quote: "If he's not interfering with play then what's he doing on the pitch?" is attributed by some to Bill Shankly.
The assistant referees' task with regards to offside can be difficult, as they need to keep up with attacks and counter attacks, consider which players are in an offside position when the ball is played, and then determine whether the offside positioned players become involved in active play. The risk of false judgement is further enhanced by the foreshortening effect, which occurs when the distance between attacking player and the assistant referee is significantly different from the distance to the defending player, and the assistant referee is not directly in line with the defender. The difficulty of offside officiating is often underestimated by spectators. Trying to judge if a player is level with an opponent at the moment the ball is kicked is not easy: if an attacker and a defender are running in opposite directions, they can be two metres apart in a tenth of a second.
In 1848, H.C. Malden held a meeting at his Trinity College, Cambridge rooms, that addressed the problem. Representatives from Eton, Harrow, Rugby, Winchester and Shrewsbury schools attended, each bringing their own set of rules. They sat down a little after 4pm and by five to midnight had drafted what is thought to be the first set of "Cambridge Rules". Malden is quoted as saying how "very satisfactorily they worked".
Unfortunately no copy of these 1848 rules exists today, but they are thought to have included laws governing throw-ins, goal-kicks, halfway line markings, re-starts, holding and pushing (which were outlawed) and offside. They even allowed for a string to be used as a cross bar.
A set of rules dated 1856 was discovered, over a hundred years later, in the library of Shrewsbury School. It is probably closely modelled on the Cambridge Rules and is thought to be the oldest set still in existence. Rule No. 9 required 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 opponent's goal; the Football Association also compromised slightly and adopted the Cambridge idea of three. Finally, Sheffield came into line with the F.A., and "three players" were the rule until 1925.
The change to "two players" rule 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.
Throughout the 1987–88 season, the GM Vauxhall Conference was used to test an experimental rule change, whereby no attacker could be offside directly from a free-kick. This change was not deemed a success, as the attacking team could pack the penalty area for any free-kick (or even have several players stand in front of the opposition goalkeeper), and the rule change was not introduced at a higher level.
In 1990 the law was amended to consider an attacker to be 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.
The offside trap is a defensive tactic designed to "trap" the attacking team into an offside position. When an attacking player is making a run up the field with a team-mate ready to kick the ball up to him, all the defenders (except the goalkeeper) will move up-field in a relatively straight line in order to put the attacker behind them just before the ball is kicked, hence putting the attacker in an offside position when the ball is kicked. Defenders using this tactic often attempt to bring an attacker's potential offside status to the attention of the assistant referee, typically by shouting or raising their arm.
The use of the trap can be a risky strategy as all the defenders (except the goalkeeper) have to move up together in a relatively straight line, otherwise the attacking players will not be in an offside position as long as they are behind the goalkeeper and a defender that has not moved up; if the offside trap fails, the attacking players will have an almost clear run towards the goal. The 2003 rule changes have made it even more perilous as a tactic, since the definition of active play was made more stringent. Thus, teams attempting an offside trap are less likely to have an offside offence called when they have caught a player in an offside position if he is deemed by the referee to be not in active play.
This defensive tactic was parodied in the film, The Full Monty, when unable to step forward in a line in a dance move the character Horse (Paul Barber) suggests simply emulating the Arsenal offside trap: 'It's the Arsenal offside trap [...] Lomper here is Tony Adams right. Any bugger looks like scoring, we all step forward in a line, and wave our arms around like a fairy' prompting the reply 'Ah well then, that's easy'.