What it is

A false start is a foul by an offensive player who moves before the snap in a way that looks like the start of the play.

In a game

It is a five-yard penalty, and the play is stopped at once. Linemen in their stances must not make a sudden move, and the offense has to stay still until the ball is snapped.

More detail

False starts are most common in loud stadiums, where the crowd makes it hard to hear the snap count. A false start can also tempt defenders to cross the line, so a defender sometimes jumps in reaction. Offenses use a silent count, with signals instead of a spoken call, to avoid the problem on the road.

Animated diagram

1. The offense lines up and waits for the snap count. Everyone has to stay still until the ball is snapped.
2. A tackle moves before the snap, and a defender reacts to it by jumping across the line.
3. The officials stop the play and call a false start on the offense. The penalty is five yards.