Defensive fronts
Run gaps (A, B, C, D)
The lanes between offensive linemen: A between center and guard, B between guard and tackle, C outside the tackle, D outside the tight end.
Gaps are lettered outward from the ball. The A gaps sit either side of the center, the B gaps between guard and tackle, the C gaps outside the tackles, and the D gaps outside the tight end.
Every run defense is a gap assignment: eleven defenders covering every lane. A missed fit, not a missed tackle, is what turns a four-yard run into a long one.