A foul line is a boundary line in sports that separates the playing area from out-of-bounds territory. Cross it at the wrong time, and your play might not count.

In baseball, the two foul lines run from home plate along the first and third base sides, extending all the way to the outfield fence. A batted ball that lands inside these lines is fair, while one that lands outside them is foul. Outfielders often make spectacular catches right at the line, with one foot in fair territory and one in foul.

In basketball, the foul line is where players stand to shoot free throws after being fouled. It sits 15 feet from the basket, and players must keep their feet behind it when shooting. Step on or over the line before the ball hits the rim, and the shot might not count, even if it goes in.

The term captures something important about rules in general: clear boundaries help everyone know what's allowed and what isn't. Whether you're shooting free throws or tracking a fly ball, the foul line can make the difference between a successful play and one that has to be done over.