Prior to the start of a play in American football, if you position yourself illegally, your team can be assessed an offside penalty. The term offside comes from the fact that football players line up across from each other on one side of the ball or the other. A mainly invisible line of scrimmage separates players, who must stay on their team's side of that divider until the play starts.
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To start a play, players line up facing each other. The offense's center lines up over the ball, which must remain on the ground, placing his hand or hands on the ball. The line from one side of the field to the other that bisects the football is known as the line of scrimmage; it is mainly invisible if it doesn't coincide with a stripe that delineates a certain yard line, such as the 20-yard line. Defensive players must line up in front of the ball, meaning the ball must be between them and the offensive team. Offensive players must stay behind the ball on their side.
Starting the Play
Once the center snaps the ball, lifting it and moving it backward to the quarterback or a running back, the play starts and players may cross the line in either direction. Offensive players who move forward in their stance before the ball is snapped in an attempt to draw defensive players offside can receive a penalty, even if they do not cross the line of scrimmage. Examples of illegally attempting to draw a defensive player offside include the center rocking the ball before he snaps it, the quarterback moving his head backward or a lineman moving his body forward.
Moving Offside Penalty
If a player moves across the line of scrimmage before the ball is snapped, the player is offside. That player's team receives a penalty, usually 5 yards. The referee moves the ball 5 yards closer to the defense’s end zone if a defensive player is caught offside, and 5 yards back toward the offense’s goal line if one of that unit's players is offside.
Stationary Offside Penalty
Players may not line up even with the ball. The area that is the length of the ball, from tip to tip, is the neutral zone; it extends from one side of the field to the other. If a player lines up with any part of his body in the neutral zone and is still there when the ball is snapped, the referee can stop the play and issue an offside penalty. During kicks, offensive players must stay behind the ball, too.
If a defensive player moves across the line prior to the snap, but does not make contact with an offensive player or cause him to move offside and retreats back to his side of the ball before the snap, there is no offside penalty. An alert center will snap the ball, even before the agreed-upon snap count, if he sees a defensive player move offside. If the center can snap the ball before the defensive player can retreat, the play starts — and the referee can then stop play and issue an offside penalty against the defender.