American Football (NFL) in : The down-and-distance system

The objective is simple to state and hard to execute: move the ball into the opponent end zone more often than they move it into yours. Possession is governed by the down system. The offense has four attempts, called downs, to advance the ball at least ten yards. Succeed, and it earns a fresh set of four downs, a first down. Fail, and possession turns over to the defense at the spot of the ball. Because handing the opponent good field position is costly, teams almost always punt on fourth down unless they are close to the marker or already in scoring range.

A game is sixty minutes of play divided into four fifteen-minute quarters, but the clock stops frequently, on incomplete passes, out-of-bounds plays, and scores, so a broadcast runs roughly three hours. Play begins with a kickoff and proceeds snap by snap from the line of scrimmage, the imaginary line where the ball is spotted. Each play starts when the center snaps the ball to the quarterback. The team with more points when time expires wins; a tie sends the game to overtime. Officiating is handled by a seven-person crew, with instant replay available to review scoring plays, turnovers, and other reviewable calls.

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.