American Football (NFL) in 1920: A professional league is founded

Every play begins from the line of scrimmage, an imaginary line running the width of the field through the spot of the ball. The offense must line up at least seven players on that line, and the play starts when the center snaps the ball backward between the legs, usually to the quarterback. Until the snap, no offensive player except one man in motion may move. The defense lines up on its own side of a neutral zone the length of the ball, and neither team may cross into it before the snap without drawing a penalty.

The offense is given four downs to advance the ball ten yards from where the series began. The situation is described as down and distance, for example first-and-10 or third-and-2. Gaining the ten yards resets the count to first down and awards a new set of four. Fail on all four, and the defense takes over at the dead-ball spot, called a turnover on downs. This single mechanic drives nearly all strategy: teams weigh running to grind out short yardage against passing to gain chunks, and the chains, a physical ten-yard measuring device, settle close calls.

On each play the offense may throw one forward pass, and only from behind the line of scrimmage. Eligible receivers, chiefly wide receivers, tight ends, and running backs, may catch it; interior linemen generally may not. A completion counts once a receiver controls the ball and gets both feet, or another body part, down inbounds. If the pass hits the ground first it is incomplete, the clock stops, and the down is used with no gain. A defender who catches a forward pass records an interception and immediately becomes a ball carrier, a live turnover that can be returned for yards or a score.