In football (soccer), a blue flag is commonly used by assistant referees (also known as linesmen) to indicate that the ball has gone out of play for a corner kick. The flag is typically blue and triangular in shape, and is waved vigorously to get the attention of the referee and players on the field.

Corner kicks occur when the ball crosses the goal line after being touched last by a defending player. The attacking team is then awarded a corner kick, which they can use to attempt to score a goal. The blue flag is used to indicate that the ball has gone out of play and the corner kick should be awarded.

In addition to indicating corner kicks, assistant referees also use the blue flag to indicate when the ball has gone out of play for a throw-in or a goal kick. The flag is an important tool for officials to ensure that the game is played fairly and according to the rules.