The area outside the field of play. The boundary is marked by a large white line that surrounds the perimeter of the playing field. A player is considered to be out of bounds when any part of his body makes contact with the white line. Play stops when that occurs. On offense, a receiver catching the football must get two feet in the field of play after securing the ball before going out of bounds for the catch to be valid.
Out of bounds is a significant part of offensive and defensive strategy. On defense, players will use the sideline as an extra defender by guarding the inside of the field. When a player is running with the ball, defenders may choose to push the player out of bounds rather than tackling him in order to stop play. On offense, out of bounds plays a crucial part in managing the clock late in each half. Teams will run plays with the intent of running out of bounds because the clock stops upon contact with the boundary. In these situations, the defense will attempt to tackle the offensive players in bounds rather than letting them get to the sideline and stop the clock. Also, over the course of play, a player who goes out of bounds cannot be the next player to touch the ball. Doing so results in a penalty.