The line judge is one of few officials that officiates a football game. The line judge stays close to the sidelines, working together with the down judge, who stays in the opposite side in the same position. The line judge has to look out for different kinds of faults as a play starts and develops, with a focus on the area around the line of scrimmage and the nearest sideline.
Before the snap line judges will assist the head linesman in looking for faults committed near the line of scrimmage, such as illegal motions, offsides, false starts, and encroachment; all very common faults committed by players.
After the snap, line judges will keep an eye at the football. If a pass is made in the play, the line judge needs to make sure it was made before the line of scrimmage, and rule if it was a forward or lateral pass. In addition, line judges have to make sure that a player was not out of bounds when running with the ball or receiving a pass. Calls like that can completely change a game, making it necessary for those officials to be very attentive.
In field goal attempts and punts, the line judge has a similar task, to make sure the kick was made behind the line of scrimmage.