In association football, an assistant referee (AR, known as a linesman or lineswoman before 1996, expressions which are still in common unofficial use) is an official empowered with assisting the referee in enforcing the Laws of the Game during a match. Although assistants are not required under the Laws, at most organised levels of football the match officiating crew consists of the referee and at least two assistant referees. The responsibilities of the various assistant referees are listed in Law 6, "The Other Match Officials". In the current Laws the term "assistant referee" technically refers only to the two officials who generally patrol the touchlines, with the wider range of assistants to the referee given other titles.

The assistant referees' duties generally consist of judging when the ball has left the field of play – including which team is entitled to return the ball to play, judging when an offside offence has occurred, and advising the referee when an infringement of the Laws has occurred out of his or her view. These two officials are typically positioned on opposite touchlines, and each stay beside different halves of the pitch.

At higher levels of play the referee is also assisted by a fourth official. The fourth official's duties are usually administrative in nature, such as supervising the substitution procedures, and he or she will generally spend the game in the vicinity of the teams' technical areas.

Competition rules will mandate procedures for replacing officials who are unable to continue. Often, the fourth official will replace the referee or one of the assistant referees if they are unable to continue. The Laws also allow for designed reserve assistant referees who have no other responsibilities unless called upon to replace a member of the officiating team who is unable to continue.

At very high-level games there may be further assistant referees. Additional assistant referees are positioned to observe incidents near the two goals. Video assistant referees view footage of the game and can advise the referee to review decisions that may have been incorrect.

The referee is the sole arbiter of the Laws during the match. Assistants' calls and judgements are considered to be advisory and can be overruled by the referee.

Laws of the Game (rules of association football)

Law 1: The Field of Play · Law 2: The Ball · Law 3: The Players · Law 4: The Players' Equipment · Law 5: The Referee · Law 6: The Other Match Officials · Law 7: The Duration of the Match · Law 8: The start and restart of play · Law 9: The Ball In and Out of Play · Law 10: Determining the outcome of the match · Law 11: Offside · Law 12: Fouls and Misconduct · Law 13: Free kicks (direct and indirect) · Law 14: The Penalty Kick · Law 15: The Throw-In · Law 16: The Goal Kick · Law 17: The Corner Kick

Comparisons
Football and Rugby Union · Football and Futsal IFAB
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