What it is

The line of scrimmage is the imaginary line across the field where the ball sits before the snap. The offense and the defense line up on opposite sides of it.

In a game

After a play the ball is placed where the runner was stopped, and that spot becomes the new line of scrimmage.

More detail

The neutral zone is the space between the two ends of the ball. Defenders may not enter it before the snap, and offensive players must stay on their own side of it. Penalties such as offside and encroachment come from crossing it too early.

Animated diagram

1. The ball is placed on the field and the two teams line up on each side of it. That spot is the line of scrimmage.
2. The strip between the front tips of the ball, the neutral zone, is empty. No player may cross it before the snap.
3. When the ball is snapped, the play begins and both lines move across the line.