We’ve been talking about gaps and fronts in a lot of detail this year, but we haven’t had much of a refresher on the topic.

In a perfect defensive world, defensive coordinators wouldn’t have to worry about defending gaps because their guys would destroy every blocker and get five guys running wild in the backfield on every snap. But things don’t work that way, so defenses hedge their bets by assigning a defender to every gap—both existing and potential—on the field.

Gaps are just the empty spaces between adjacent offensive players. Because they’re empty spaces, every gap is a potential path for a runner. That’s why most defensive calls have a defender assigned to every gap (the main exceptions are pass blitzes and deep/prevent-style coverages.) “Gap” here is a defensive term; when offenses talk about these spaces they call them “holes” and assign numbers to them. Here’s a mock-up of how gap assignments can look for box defenders against a spread formation:

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