Baseball fields have several distinguishing markings to indicate what is in play and what is out of play. The most prominent of these markings is the foul line, which runs from home plate to the outfield fence in a straight line on both the right side of the field and on the left. The infield portion of this line – running from home plate to first base, and another from home plate to third base – is called the baseline and it indicates where the runner must run in order to reach base. There is an imaginary baseline between first base and second base, and another between second base and third base. If a runner runs outside of the baseline – usually within three feet of center – he is called out by the umpire.

Foul lines are usually chalk markings, but many ballparks now use white spray paint to lay the lines. Because dimensions of the field vary depending on the size of the ballpark, the foul lines vary in length. But they always begin at the corner of the batter’s box and run all the way to the outfield fence. The foul line – which includes the first baseline and third baseline – indicates whether a ball is in play or out of play. If the ball lands in foul territory, it is dead and play stops. Often a batted ball rolls along the baseline, and a fielder must wait to see if the ball stays fair or rolls foul. In order for the ball to be fair, it must pass either first or third base in fair territory.

Runners are susceptible to running out of the baseline when avoiding a tag from an infielder. Because there is no official marking aside from the baseline on the first base side and the one on the third base side, an umpire must then make a judgment call as to whether a runner has gone too far outside of the baseline. This is especially difficult when a runner is running between first and second base, or second and third base, as there are no chalk markings by which an umpire may gauge a runner’s distance from the baseline. The umpire may deem a runner’s avoidance of a tag within reasonable distance of the baseline (usually within three feet), or he may call the runner out for being too far from the baseline while attempting to avoid a tag.