Law 6 - Linesmen

Two linesmen shall be appointed, whose duty (subject to the decision of the referee) shall be to indicate:

when the ball is out of play
which side is entitled to a corner-kick, goal-kick, or throw- in,
when a substitution is desired.

they shall also assist the referee to control the game in accordance with the Laws. In the event of undue interference or improper conduct by a linesman, the referee shall dispense with his services and arrange for a substitute to be appointed. (The matter shall be reported by the referee to the competent authority.) The linesmen should be equipped with flags by the club on whose ground the match is played.

Decisions of the International F.A. Board

Linesmen, where neutral, shall draw the referee's attention to any breach of the Laws of the Game of which they become aware if they consider that the referee may not have seen it, but the referee shall always be the judge of the decision to be taken.
National Associations are advised to appoint official referees of neutral nationality to act as linesmen in international matches.
In international matches linesmen's flags shall be of a vivid color, bright reds and yellows. Such flags are recommended for use in all other matches.
A linesman may be subject to disciplinary action only upon a report of the referee for unjustified interference or insufficient assistance.