What it is

A gap is the open space between two neighboring offensive linemen. The gaps along the line are named with letters, starting from the middle: the A gaps are on either side of the center, the B gaps are between each guard and tackle, the C gap is between the tackle and the tight end, and the D gap is outside the tight end.

In a game

On a running play every defender has a gap that he must protect, which is called his gap responsibility. If a defender leaves his gap to chase the ball, the runner can cut into the empty space, so each one holds his gap even when the play seems to be going somewhere else. Linebackers and safeties fill gaps too, once the ball carrier picks one.

More detail

Gaps are also how coaches describe a defensive plan. A lineman in a “two-gap” system lines up in front of a blocker and controls both gaps beside him, while a “one-gap” lineman attacks a single gap and trusts his teammates to cover the rest. The offense thinks about gaps too: the runner looks for the one that opens, and gap blocking sends linemen to wall off a gap. Pass rushers also attack gaps, because a gap is the shortest path to the quarterback.

The diagram

1. The gaps are the spaces between the offensive linemen. The ones beside the center are the A gaps.
2. Moving outward, the next spaces, between the guard and the tackle, are the B gaps.
3. Between the tackle and the tight end is the C gap. Outside the tight end is the D gap.
4. On a run, each defender is responsible for one gap. If he leaves it, the runner can cut through.