In baseball, the foul lines are a feature of the field that comprises the outer boundaries of the baseball diamond, stretching the length of the field to the fence or the wall encompassing play.
The foul lines are drawn in chalk and stretch in straight lines to the right and left at a 45-degree angle from home plate. Both lines follow the outside edges and continue past first and third base, respectively. The space inside the foul lines, including the foul lines themselves, delineates fair territory, while the space outside the lines is considered foul territory. When a ball is put into play by a batter, the foul lines determine whether the ball has been hit fair or foul. In the case of a ground ball, if the ball first strikes the ground in foul territory or crosses the line into foul territory before passing a base, it is considered a foul ball and the play is called dead. On the other hand, if a fly ball is caught in foul territory, the result is a putout.
In the case of a ground ball passing over first or third base and landing in fair territory, the ball will only be called fair if the umpire determines that the ball crossed the base on the fair side of the foul line (including on the foul line itself).