Two of the players at the line of scrimmage must be receivers –either wide receivers, tight ends or even the occasional running back– and they have to be lined up before the play can begin. All other offensive team members, including the quarterback, are in what is called the backfield, or behind the line of scrimmage.

Both teams must remain on their side of the line of scrimmage prior to the ball being snapped. A penalty called a false start will be called if an offensive player crosses the line prior to the snap of the ball. Offside is a penalty against a defensive player that is across the line of scrimmage when the ball is snapped.