A simple single-gap 4-4 front versus an Ace set; notice how every defender covers an initial gap.

What is a “Gap-Sound” Defense? Pt. 1

Pretty early in football’s evolution, coaches realized that even the simplest, most repetitive run game could be devastating if a defense wasn’t “gap-sound.”  Gaps—the spaces between offensive players—are the high ground of football: whoever controls them usually controls the game. If a defense leaves a gap open at the snap, any decent tailback will see the vacancy and run for a gain. Even if the defense initially covers a gap, leaving too soon invites cutbacks, counters, and misdirection plays.

Defenders are now assigned gaps to cover. These assignments can change based on play-call and what the offense does. In a 4-4 defense, the gap assignments might look like this:

A simple single-gap 4-4 front versus an Ace set; notice how every defender covers an initial gap.

Notice how each gap (or hole, if you’re looking at it from the offense’s perspective) has an initial defender, making this a purely one- or single-gap defense. In the base scheme above, defenders are aligned close to the gap they’re supposed to control: the linemen are shaded to make it easier to defeat blocks, while the linebackers are standing directly over their gaps so that they can rush in and make a play with minimal impedance. Being positioned over your assigned gap also makes reading and reacting to plays easier. If a ball-carrier runs into one of these gaps, the assigned defender should make the tackle for a loss or minimal gain.

How the defense defends these gaps depends on a lot of factors. Players on the line of scrimmage generally either attack the nearest blocker in order to gain leverage, or try to beat the blocker off the snap by exploding into the gap itself. Players off the line of scrimmage (the four linebackers in this case) might try to meet any blockers or runners in the hole, creating a pile-up in the first case and notching a tackle in the second.

Knowing when and how to abandon an assigned gap is a critical skill. Defenders are taught how to recognize if their gap is threatened, and if no one is coming to vacate the gap and help in pursuit or to defend against cutbacks and misdirection plays. They do this not just by watching the running backs, but by watching where and how the linemen are blocking. Middle linebackers, for example, are often assigned to read the center and both guards, while also being aware of the backs. If one guard pulls while the center and second guard aggressively block down, for instance, the Mike is probably worrying about a Power play. On the other hand, if all three take bucket steps in the same direction as the back is running, then he might be more worried about a zone run.

An offense can scheme to take advantage of a defense’s gaps assignments. Isolations and pulls are common ways of complicating gap assignments, as are option plays. When it’s clear which gap (or gaps) is actually threatened (or threatening to appear), defenders away from the play switch to defending against the runner or lead blockers. Defenders with assignments like “force,” “spill,” and “contain” try to either corral the runner back to the strength of the defense or towards the sideline by aggressively meeting blockers and option men. “Fill,” “cutback,” and “insurance” assignments make clean-up tackles against lead plays or provide backup when the defense breaks down. (You’ll see “contain” used in this context, too.)

Defenses that become too predictable in their gap assignments can make themselves vulnerable. Offensive coordinators can recognize the scheme and either throw shifts and formations that force the defense into tough positions by scheme, or they can single out weak links on the offense to attack. In addition to simply changing formations and assignments to vary the defense, blitzes, shifts, and exchanges are great ways to keep offenses off-balance. Blitzes, especially from unexpected defenders, can confuse blockers and bait running backs into traps. Last second shifts of the front (or “stems”) force blockers on gap-scheme runs to pick new assignments. Exchanges, where defenders move after the snap to cover more distant gaps, are subtler ways of getting to an offense.

Probably the biggest disadvantage of the purely single-gap scheme is its deficit versus the passing game. In today’s game, it’s nearly impossible to run a single-gap 4-4—there are just not enough people in the secondary. Moving one guy back to give you two safeties while still having him directly responsible for a gap is doable but requires a special player. This defender has to be smart enough to not only play the pass while watching a gap, but have the physical tools to do both. Undersized safeties without a nose for tackling will get targeted by opposing offenses, while stiffer, slower run-stuffers are prone to giving up deep balls.

Even with an excellent player, though, advanced passing schemes can create tremendous problems for a pure single-gap front. An answer to this dilemma is to have players who are in some way responsible for two gaps, though as we’ll see in the next installment this isn’t a perfect solution.

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