In baseball, foul lines are field markings which define the boundary between fair territory (the playing field) and foul territory (out of bounds). They are used solely to determine whether or not a ball hit by a batter is to be considered a foul ball.

Extending from home plate, past first and third bases all the way to the far walls in the outfield, foul lines are three inches wide, white and made of paint, crushed chalk or lime. Chalk foul lines last longer (typically five to ten times) if they are lightly dampened with water immediately after they've been laid.

mcSey points out that many superstitious batters consider stepping on a foul line when entering or leaving the field bad luck.