The offside rule means a player who is offside is committing a foul, unless he is deemed to be not interfering with play (eg, on the other side of the pitch and consequently unable to receive a pass). In particular, a goal may be disallowed because a player was offside during set up. The other team is rewarded with an indirect free-kick taken from the place the offside player was standing.
In enforcing this law, the referee depends greatly on his assistants (also known as linesmen), who generally try to keep in line with the last defender (not counting the goalie).
The offside rule is often cited in the UK as something women are unable to understand. However, many men also have a shaky grasp of details of the law.
History
It is often assumed that the offside law is a recent addition to combat "goal scrounging" or "cherry picking", where attacking players hang around near the opposing goal in case the ball gets kicked upfield, but in fact it dates back to the early years of the game, and was much stricter in the past than it is today.
A player was "off his side" if he was standing in front of the ball (compare with the current offside law in rugby - a game descended from the same roots), that is, between the ball and the opponent's goal.
This was by no means universal - the Sheffield Association had no offside rule, and players known as "kick throughs" were positioned permanently near the opponents' goal.
In 1848, in a meeting at his Trinity College rooms, HC Malden held a meeting to address the problem. Representatives from Eton, Harrow, Rugby, Winchester and Shrewsbury 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 offside, throw-ins, goal-kicks, halfway line markings, re-starts, and the disallowing of holding and pushing. They even allowed for a string to be used as a cross bar.
Slowly, as these rules were tried, tested, written and re-written over the following years, a revised set of Cambridge Rules was drawn up in 1856. A copy of these rules, thought to be the oldest set still in existence, can be found in the Shrewsbury School library.
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 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 lead to an immediate increase in goal scoring. 4,700 goals were scored in 1848 Football League games in 1924/25. It rose to 6,373 goals (from the same number of games) in 1925/26.
more stuff goes here
In 1990 the law was amended to allow a player to be played onside by someone in line with him. This change was part of a general movement by the game's authorities, in the early nineties, to make the rules more conducive to attacking football and help the game to flow more freely.
The offside trap is a defensive tactic, for which Arsenal are particularly famed.
If an attacking player is making a run up the field with another player ready to kick the ball up to him, then the defenders will move up-field, putting the attacker offside.
The use of the trap is often derided as making for boring football.The offside trap