Every Friday, former NFL defensive back Matt Bowen will take you inside the playbook with his “NFL 101” series. An in-depth look at personnel, formations, coverages, route concepts and more to give you a better understanding of the game. Today, he breaks down the basics of the 4-3 defensive front.

Personnel ID

In the 4-3 front, there are four defensive linemen and three linebackers. Here is a quick breakdown of how you can identify the personnel in the playbook.

Closed (strong-) side end: DE

Open (weak-) side end: DE

3-technique tackle: DT

Nose tackle/Nose guard: N (or NT)

Weak-side linebacker/"Will": W

Middle linebacker/"Mike": M

Strong side linebacker/"Sam": S

4-3 Alignments/Gaps

Before we get to the chalkboard and All-22 looks of the two base 4-3 fronts (“Over” and “Under”), we need to discuss the gaps and alignments along the defensive line.

The 4-3 front  is a “one-gap” scheme (this will change for “hybrid” fronts where players will use a “two-gap” technique). Each defender in the front (both seven-/eight-man fronts) is responsible for a gap versus the run game. These gaps are identified by letters (A,B,C, etc.).

As you can see in the diagram, the gap (or hole) between the center and the guard is the "A-gap." This identification system works inside-out, with the next gap (between guard and tackle) labeled as the “B” gap, followed by the "C-gap" (between tackle and tight end) and the outside "D-gap" (edge contain).

The next step is the alignments, done with a numbering system to identify the “techniques” of the defensive lineman. The defense counts inside-out, starting at the "0-technique" (over the center) and finishing with the “9” (or “Wide 9”) along the front.

The Nose will usually align in the “0 shade” (offset to the open/closed side of the formation over the center) or in a “2i” (inside shoulder of the guard) with the defensive tackle in a “3-technique” (outside shoulder of the guard to the closed/open side).

At the defensive end positions (both open and closed) the alignments, or “techniques,” will vary based on game plan. You will see the ends align in a “5,” “6,” “7” and the “9.”

“Closed” vs. “Open”

When the offense breaks the huddle and lines up, the defensive will then “set” the front with a “closed” call to the tight end side. If two tight ends are in the game, the defensive coordinators will identify which tight end they are going to close the front to during game-week prep.

Here’s an example of the Cowboys' "4-3 Under" front with a “closed left” call (to the tight end). The Sam ‘backer aligns to the “strength” of the formation with the Nose in a “0 shade,” and the defensive tackle (“3-technique”) now aligned to the “open” (or weak) side in the Under front.

The “closed right/left” call is made by the Mike ‘backer, and the defense will adjust their alignment (or “technique”). This is very important from a secondary perspective for the strong safety and free safety when setting an eight-man front.

As you can see in the All-22 picture above, the Cowboys are walking a safety down into the box to the open side of the formation (away from the tight end). This huddle call is “4-3 Under 10,” where the safety tracks the fullback versus a weak-side run and plays the open-side A-gap on run away (cutback).

4-3 “Over”

In the “Over” front, the “3-technique” defensive tackle is aligned to the closed side of the formation (tight end side) with the Nose aligned either in a “0 shade” or a “2i” to the open side.

The linebackers will use a “tan-zero-tan” stack look (tackle-zero technique-tackle) with the closed-side defensive end in a "7-technique" (outside shoulder of tight end) and the open-side end in a variety of techniques (5-7-9) based on game plan.

Here’s a static, chalkboard look at the "4-3 Over" front with a “closed left” call versus Regular/21 personnel (2WR-1TE-2RB) in a Pro Strong I formation.

Using a Cover 2 shell in the secondary, I have the linebackers (S-M-W) in the “tan-zero-tan” alignment with the “3-technique” tackle (DT) to the closed side of the formation and the Nose (N) in a “0 shade.”

Now let’s focus on the gap “fits” (remember, this is a one-gap front) using the All-22 film from the Bears-Vikings matchup with the offense in a Pro I formation.

With a "closed left" call (tight end side), we can see how the Bears' front seven fits versus the run. To the open side, the defensive end plays the "C-gap" with the Will ‘backer in the "B-gap" and the Nose (“0 Shade”) in the “A-gap."

To the closed side, the defensive end plays the "D-gap" with the Sam ‘backer in the "C-gap," the “3-technique” defensive tackle in the "B-gap" and the Mike ‘backer in the "A-gap."

4-3 “Under”

The key to knowing the difference between the “Over” and “Under” front in the 4-3 defense is the alignment of the “3-technique” defensive tackle and the Nose.

In the “Under” front, the Nose is aligned to the “closed” side of the formation with the “3-technique” defensive tackle aligned to the “open” side—opposite of what you see in the “Over” front.

Here’s how the 4-3 Under front is drawn up on the chalkboard versus Regular/21 personnel (2WR-1TE-2RB) in a Pro Strong I formation with a “closed left” call.

With the Nose (N) aligned to the closed side of the formation, and the Sam ‘backer (S) now walked up over the tight end, the “3 technique” defensive tackle (DT) moves to the open side.

At the second-level, both the Mike (M) and Will (W) ‘backers shift to the “bubbles” (uncovered gaps) with the closed side defensive end (LE) kicking down inside over the right tackle.

Just as we did with the “Over” front, let’s look at the gap fits in the “Under” front using the All-22 tape from the Bears-Vikings matchup at Soldier Field with the offense in a Pro I “Z Nasty” formation.

With the Bears in a “closed right” call, let’s take a look at the open side of the formation. The open-side end plays the “C-gap" with the “3-technique” defensive tackle in the “B-gap" and the Will ‘backer filling the "A-gap."

To the closed side, the Sam’ backer fits inside the Z receiver (nasty or reduced split) in the “D” gap with the closed side defensive end in the "C-gap," the Mike ‘backer in the "B-gap" and the Nose (aligned in a “0 shade”) in the "A-gap."

The “Wide 9” Alignment

We hear the term “Wide 9” often when discussing the 4-3 front, but what exactly does that mean?

Remember the “techniques” we talked about above in the first diagram? The "9-technique" is used to identify the alignment of the defensive end in a 4-3 that is widened from the core of the formation.

Here’s a shot of  the Lions “Wide 9” look ("Over" front) versus the Bengals Pro I formation out of Ace/12 personnel (2WR-2TE-1RB) with tight end Tyler Eifert shifted into the backfield as the “F” (or fullback).

Look at both defensive ends. Not only are they removed in the "9-technique" alignment, but they've also adjusted their stance (shade).

Inside, the Lions have the "3-technique" defensive tackle to the closed side of the formation (“closed right” call) with the Nose in a “2i” to the open side and the linebackers in the “tan-zero-tan” alignment.

Personnel Fits/Keys to the 4-3 Front

If you want to run a 4-3 front, what are the key positions to focus on? And what type of skill set are you looking for at those spots? Here are the four positions, starting with the "3-technique" defensive tackle, that I would point to first when building a front seven in the 4-3.

—"3-Technique" defensive tackle

Speed/quickness up the field with the ability to use technique/power at the point of attack to hold the gap. Think of the Bucs’ Gerald McCoy and the Bengals' Geno Atkins or look to this year’s draft class with Pittsburgh’s Aaron Donald and Florida State’s Timmy Jernigan.

—"Open-side" defensive end

The majority of NFL offenses are right handed in terms of formation and alignment, so I look to the open-side end (or right end) as a key factor in the 4-3 front.

You want size, length and true athletic ability on the edge of the defense that can hold the point, turn the corner versus an offensive tackle and play the cutback against the run game. That’s the Rams’ Robert Quinn or the Giants’ Jason Pierre-Paul with Jadeveon Clowney up next in this year’s draft class.

—Will ‘backer

In the 4-3, the Will ‘backer is going to pursue to the football, shoot backside gaps and clean up on cutback runs. He is the “playmaker” at the second level for defenses that run the 40 front. Again, think about athletic ability (versus both the run and the pass) with the Bucs’ Lavonte David or the Bears’ Lance Briggs as prime examples of the skill set you want at the Will 'backer position.

—Mike ‘backer

The Mike ‘backer has to showcase change of direction skills (hip transition), speed down the field in Cover 2 and also fill the hole as a downhill defender versus the one and two-back power-run game. That was Brian Urlacher in the past. And in today’s game, I look at the Panthers’ Luke Kuechly and the Cowboys’ Sean Lee.

Up Next in the “NFL 101” Series: The 3-4 Front

Seven-year NFL veteran Matt Bowen is an NFL National Lead Writer for Bleacher Report. 

Follow @MattBowen41