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A Linesman in the game of Soccer, is a member of staff on the pitch who works with the Referee to ensure the smooth running of the game. Usually there are 2 linesmen, one on either side of the pitch. Their job is to indicate to the Referee that a ball has gone out of play, either over the sideline or through the corner of the side they're monitoring, or to indicate that a player is offside during the course of play. In the event that for whatever reason, the referee is unable to continue controlling the game, either through illness or injury, one of the linesmen may be called on to take charge as a substitute.