The rules in football allow each team to have eleven players on the field at a time. Teams may substitute players between plays with no restrictions. Each team must start a play on their side of the ball.
The defensive players may take any position they want and can move about their side of the football prior to the play without restriction. Although there are certain defensive positions that have become common over time, there are no specific rules defining defensive positions or roles.
The offensive players, however, have several rules that define their position and what role they may take in the offense. Seven offensive players must be lined up on the line of scrimmage. The other four players must be lined up at least one yard behind the line of scrimmage. All of the offensive football players must be set, or still, prior to the play beginning with the exception of one of the four backs which may be moving parallel or away from the line of scrimmage. Further rules say that only the four backs and the players at each end of the line of scrimmage may catch a pass or run the football.