What it is

The snap is the moment the center passes the ball back to the quarterback to start a play. Neither side may cross the line of scrimmage before it.

In a game

Offenses use a snap count, a signal the quarterback calls, so the whole line moves at the same instant. A defender who jumps early is offside.

More detail

A snap can be taken under center or in the shotgun, and a bad snap can lose the ball or yards. The offense also uses a hard count, a sudden change in the rhythm of the snap count, to try to make a defender jump offside. After the snap, the ball is live and the play is on.

Animated diagram

1. Before the snap the quarterback stands right behind the center and calls the count.
2. On the snap the center passes the ball back through his legs into the quarterback’s hands.
3. The whole line fires out at the same moment, because the count tells every lineman when to move.