Offside

Officiating
An assistant referee signals that the offside offence was in the middle of the pitch; on the far side the flag would be pointed up at 45 degrees, for near the assistant it would be pointed down.

In enforcing this rule, the referee depends greatly on an assistant referee, who generally keeps in line with the second-to-last defender, the ball, or the halfway line, whichever is closer to the goal line of his relevant end. An assistant referee signals that an offside offence has occurred by first raising his or her flag upright without movement and then, when acknowledged by the referee, by raising his or her 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;
Flag parallel to the ground: offence has occurred in the middle third of the pitch;
Flag pointed at a 45-degree angle upwards: offence has occurred in the third of the pitch furthest from the assistant referee.

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 and when the offside-positioned players become involved in active play. The risk of false judgement is further increased by the foreshortening effect, which occurs when the distance between the 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.

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.