In soccer, the offside rule states that when an offensive player is on the opponent's side of the field and a teammate touches the ball, there must be at least two opposing players who are closer to the goal line than the offensive player is, or that player cannot become involved in the play. For example, a player who has only the opposing goalkeeper between him or her and the goal cannot receive a pass from a teammate. The two defenders closer to the goal line usually include the goalkeeper, but that it not necessarily true. When an official calls a team for being offside, the other team is given possession of the ball for an indirect free kick.
Timing is Crucial
This rule was created to prevent offensive players from "cherry picking" near the opponents' goal. Without the rule, offensive players could hover near their opponents' goal even when the play is on the other side of the field, with the hope of a long pass and an easy goal. The offside rule applies at the moment the ball is touched or passed, not when the ball is received. Therefore, if the offensive player who will receive the pass is onside at the time the ball is touched, then runs past the defender before receiving the pass, the receiving offensive player is not offside, and the play is legal.
Setting a Trap
A common strategy for the defensive team is to play what is called an offside trap, in which the defenders try to draw the opponent offside by running forward right before a potential pass, thereby causing the furthest forward offensive player to be closer to the goal line than the second-to-last defender. The offside trap can backfire, however, if the defenders do not get ahead of the receiving offensive player before the pass is made or if the officials do not call offside. If the receiving offensive player is even — or "level" — with the defenders at the moment of the pass, the player is onside, which can result in the receiving player being wide open if the defenders continue to move away from the goal line.
There are some exceptions to the rule of offside. It cannot be called if the offensive players are on their own side of the field. Nor does it apply on a throw-in, goal kick or corner kick.
Offside might be called by either the center referee or the sideline referees. If the center referee spots the offside, he or she will blow his or her whistle to stop play. More commonly, a sideline referee will signal the offside by raising a flag, indicating to the center referee that he or she should blow his or her whistle to stop play. After offside is spotted and the center referee blows his or her whistle to stop play, he or she will place the ball at the spot of the infraction. The opposing team will then get an indirect free kick from that position.
Difficult to Call
This has been a controversial rule, particularly because a referee has some freedom of interpretation about the offensive player being involved in the play, but also because it's often difficult to judge the player's positioning at the moment of a pass. The referee must determine whether the circumstances were right to make an offside call, even if the player technically was in an offside position. A referee has flexibility in ruling that the player intended to receive the pass, was moving forward and was significantly involved in the play. This means that the offensive player must have a fair chance of scoring a goal in addition to being in an offside position.