What is the 4-3 "Under" and How Do the Ravens Use It?

It seems that, every year, there's an ongoing debate about the Ravens' front seven: Do they play a 3-4 or a 4-3? Are they switching to one or the other in the off-season? And how will that affect their defensive philosophy?

After all, the Ravens made their name as an attacking, opportunistic defense. They've intimidated quarterbacks and stifled running backs for most of their 18-year history, and fans take pride in the defensive tradition in Baltimore.

So with that in mind, I dug into the defensive front that the Ravens will use as their "base" this season: the 4-3 Under.

The 4-3 Under has a storied history that begins with Monte Kiffin, who used it to win a National Championship at Nebraska. He then took his defense to Arkansas, where it was picked up by a young graduate assistant named Pete Carroll.

Carroll eventually took the defense to the Minnesota Vikings (along with Kiffin) as well as the USC Trojans and, of course, the Seattle Seahawks. You may recall that Seattle's defense - which runs a lot of 4-3 Under - just dominated Peyton Manning in this year's Super Bowl.

Meanwhile, Kiffin also took his 4-3 Under to the Tampa Bay Buccaneers, where him and Tony Dungy paired it with a coverage scheme they called the Tampa-2 to create one of the best defenses in NFL history.

Aggressive against both the run and the pass, the 4-3 Under is a proven defensive front... and it's become the base defense in Baltimore in recent years.

What Exactly is the 4-3 Under?

Essentially, it's a 4-3 front with the personnel shifted toward the "open" or weak side of the formation (away from the tight end).

But that doesn't capture the whole picture. You see, even though it's called a 4-3 defense, the personnel tend to resemble a 3-4.  Let's take a look at it on the chalkboard and then discuss personnel and player responsibilities:















Above is the Ravens' version of the 4-3 Under. The offense's gaps are labeled with blue letters, and each defender's gap responsibility is denoted by a red arrow. As you can see, most of the defenders (aside from one of the Tackles) are responsible for just one gap. Those gap responsibilities are the reason the 4-3 Under is, in fact, a 4-3 defense.

Yet the 4-3 Under is sometimes mistaken for a 3-4, and it's easy to see why. At first glance, it looks like there are four linebackers and just three defensive linemen (like a 3-4). For clarity, here are the front seven personnel pictured above:

L = LEO, a hybrid linebacker/defensive end
T = Defensive tackle
E = Defensive end
S = SAM (strong-side) linebacker
W = WILL (weak-side) linebacker
M = MIKE (middle) linebacker

Additionally, the front actually looks like a 5-2. Typically, the 5-2 is an alignment used by 3-4 defenses, not the 4-3 (three defensive linemen and four linebackers, two of which are on the LOS).

But in the end, it's the one-gap responsibilities that define the 4-3 Under. I prefer to just think of it as a defense with 3-4 personnel but 4-3 responsibilities.

Why 1-Gapping?

The advantage of 1-gapping is that defenders are able to play aggressively off the snap against both the pass and the run. They can crash their gap and try to penetrate into the backfield no matter the play. If it's a run, they'll naturally plug the hole that the running back is trying to fit through, and if it's a pass, they'll pressure the quarterback.

Contrast this with a 2-gapping scheme, which forces defenders to play a more conservative, hold-your-ground style. Instead of attacking a gap, defenders in this scheme attack a man. And since the defenders are responsible for two gaps instead of one, they can't get upfield as aggressively.

Finally, one-gapping has an added bonus: Coaches don't need to find as many big bodies who can 2-gap along the defensive line. In a traditional 3-4, all three defensive linemen are 2-gapping, which means a coach would need to find six 2-gapping players - three starters and three backups. That's no easy feat.

The 4-3 Under Hybrid

In recent years, though, the gap responsibilities in 4-3 Under have become muddied.

You see, when Pete Carroll took the defense to USC in the early 2000s, he toyed with the idea of a hybrid 4-3 Under, which asks some players to 2-gap and some to 1-gap. This gives the defense more flexibility, as it doesn't need quite as many players to cover all of the gaps.

The result is what you see on the chalkboard above - a mostly one-gapping defense, but with a 1-technique DT who is 2-gapping on the strong side. That's what the Ravens used in 2013 and, based on their roster, what they'll continue to use in 2014.

Below is a shot of the Ravens in their 4-3 Under alignment from Week 17 of last season. I've added position titles and a list of the likely personnel who will fill these roles this coming season (note: not all of these players are pictured).

As you can see, most of the positions have clearly defined starters. Canty, Ngata, Suggs, Smith, and Mosley all projected to play in Week 1.

The position with the most uncertainty is 1-technique DT.  Last season, the Ravens tried Brandon Williams, DeAngelo Tyson, Terrence Cody, and even Haloti Ngata at this position. They struggled to find a week-to-week starter here, and it hurt their defense. If Williams, Tyson, or Jernigan can become a solid 1-tech and learn to successfully 2-gap, then the defense will improve, and both Smith and Mosley will have a shot at a really strong statistical season.

Additionally, this would let Ngata play his natural position, 3-tech DT - and that would be huge for the Ravens.

You see, most people agree that the success of the 4-3 Under hinges on the 3-tech DT, a one-gapping defensive tackle who has the ability to consistently beat the offensive guard and penetrate the backfield. This player must be both a stout run stopper and a feared pass rusher. Warren Sapp was the prototypical 3-tech DT.... but Ngata's no slouch either. Some people claim he disappeared last year or no longer "has it," but I think a full season at his natural position will prove otherwise.

Outside References

1) Inside the Playbook: Over-Under Michigan's 4-3 Defense
2) USC 4-3 Under Defense - Pete Carroll
3) NFL 101: The Basics of the 4-3 Defensive Front
4) The 4-3 Under: Defining the Defense
5) Who's Laughing Now?
6) Mixing Gap Responsibilities with the 4-3 Hybrid Defense