Tag Archives: 4-4 defense

What is a “Gap-Sound” Defense? Pt. 2

As we saw in Part 1, assigning every needed defender to a single gap can stress a defense.  One way to rectify the problem is to change the math by assigning one or more players to simultaneously cover two gaps.

You can baby-step your way into this approach.   Using the 4-4 again as an example, here’s a look at how 4-4 guru Bud Foster shifted his eight man front from the 90’s (which looked much like our example front in Part 1) into a variation on the “TNT” or “Eagle” look he called the “Tuff” front, which will look familiar to defenses NFL fans saw in the 80s.  Here’s how Foster drew it up for the Hokies:


Virginia Tech "Tuff" front; click to enlarge.

To achieve this front, the down linemen shift to the strong side, while a hybrid LB/DB called a “whip” takes the vacated weak-side end’s spot and a linebacker steps up to the line.  The result is a de facto attack-oriented 6-2 defense.  If you look in front of the “backer” position (roughly equivalent to a 4-3 SLB and labeled with a ‘B’), the defensive tackle (‘T’), nose tackle (‘N’) and right-side defensive end (‘E’) you’ll notice they have angled brackets drawn in front of them.  These represent gap assignments.  The backer, DT, and DE have single gap assignments—the C and two B gaps, respectively.  (Though it’s not shown here, the DT and DE are actually in shaded 3-techniques to help them hit their gaps.)

While these three are assigned single gaps, the nose tackle has responsibility for both A gaps, as the double bracket indicates.  This is the safest place to two-gap a defender: the center is handicapped by having to snap the ball before blocking, and teams rarely run to the A gaps because it leads them into the teeth of the defense.  Unlike a slashing single-gap defender, a two-gap defender essentially reverse blocks the man in front of him by exploding off the ball, gaining leverage, identifying the play, and then chucking the would-be blocker in order to chase down the ball carrier.

This front is great against running teams because it puts eight guys in the box while keeping the interior linebackers relatively free from contact.  Here, the rover (basically a strong safety) and mike key on the running backs nearest to them without worrying about blockers coming at them.  The scheme doesn’t, however, do as good a job with pass defense.  Walking the whip and backer down has pulled intermediate-level defenders out of pass defense, while pulling the rover down (a strong safety-type position) has made the secondary vulnerable.

One solution here is just to make more players two-gappers.  At the opposite end of the two-gap spectrum from the Tuff front are all-in defenses like the 3-4.  In this front, each down lineman is responsible for two gaps.  This gives the interior linebackers a great deal of freedom in chasing down plays, while also allowing for a balanced secondary.  Here’s a slide from a Baltimore Ravens presentation showing their base 3-4 defense.

Baltimore's base 3-4 in 2004; click to enlarge.
Note how each of the three d-linemen are lined head-up on an offensive player.  The nose tackle is in the same 0-technique from the Tuff front above, while the ends are in 4-techs over the offensive tackles.  All three players are responsible for stopping runs to their immediate left and right.  When done right, this relieves the linebackers of their immediate interior gap responsibilities.  In theory, the backers can react quicker to passes, either by dropping into coverage with greater purpose, or by tracking down flat runners and screens.

Like all things in football, however, there are tradeoffs.  The first is that it’s hard finding two-gap players.  “Truck and shuck” requires a controlled mentality, while beating offensive linemen head-up and then chasing down runners requires a freakish mix of size and athleticism. This can be a pretty apparent weakness when you see 4-3 teams in college switch to an odd front to help stop the option—the smaller DTs and DEs often get overwhelmed at the point of attack.  Second, even the best 3-4 linemen can’t consistently create a pass rush by themselves—they’re always outnumbered.  Blitzing is the obvious answer, though anytime you blitz from a 3-4, you’re essentially reverting back to a mixed or single-gap defense.  The 3-4 can make up for this by being so confusing, though it’s not a perfect solution.  Finally, in a pure 3-4 there are always two offensive linemen who are “uncovered,” i.e., they don’t have a d-lineman setting up shop in front of them.  If these guys aren’t required to help with double-teams, they essentially have a free shot on the linebackers.

In between these extremes are split-scheme defenses, which are often run as much to tailor schemes to personnel strength as they are to gain a chalkboard advantage.  Bill Belichick has been a master of this; in recent years, his squads have been combinations of 3-4 and 4-3 defenders.  To make the best of both worlds, he will split his defense down the middle and make the players on one side (say, the weak side) two-gappers, while the strong-side players are single-gap.  Perhaps the biggest weakness of this method is depth, since it’s pretty much impossible to have adequate reserves for two distinct styles of defense.

Pure two-gapping is a brute force solution to the numbers game of gap assignments, and it’s as old as the infancy of football, much the same as the pure single-gap approach. Today’s even-front teams generally prefer a more situational scheme that we’ll look at in the final post.


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.