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Getting to the quarterback has never been more important than it is today in the NFL. Modern passing schemes get rid of the ball so quickly, to so many targets, in such complicated route packages, NFL secondaries can't possibly stop them for four quarters.

That's why the best pass defense isn't defense at all: It's attacking the quarterback.

There are many different ways to bring heat: inside, outside, from the line, from the blitz, out of a 3-4 or 4-3 alignment. But what's the difference between all these? What are the advantages and disadvantages? What's the most effective way to rush the modern NFL passer?

Understanding Gaps

If you've played any of the older Madden video games that feature John Madden's commentary, you know the phrase "it all starts with the offensive line." He meant that in terms of offensive success, but defenses key off the offensive line, too:

When the offense is running the ball, the gap between every offensive lineman is daylight for a running back. Defenses have to make sure that no matter which gap the running back goes through, they have a defender ready to make the stop.

When the offense is passing the ball, though, these gaps are daylight for pass-rushers.

The spaces between the center and either guard are called the "A" gaps. The spaces between the guards and tackles are called the "B" gaps. The spaces on either side of the tackles are the "C" gaps. We can think of the gap outside of a blocking tight end as a "D" gap.

When a linebacker rushes the passer through one of these gaps, that's what kind of blitz it is. A linebacker rushing the quarterback from between the right guard and right tackle is a "B Gap Blitz."

Line Alignment & Gap Responsibility

Defenses align themselves based on the offensive line. Each lineman plays a different "technique," indexed off of the position of the offensive line:

Lining up directly across from the center, guard, tackle and tight end are the zero-, two-, four- and six-technique positions. Lining up in the gaps are the one-, three-, five-, seven- and nine-technique positions. Where you see an "i" in the alignment, that's lining up in between the offensive lineman's head and shoulder—in line with his eye.

The 3-4 Pass Rush

In a traditional 3-4, the nose tackle plays a "zero technique" and is responsible for covering both A gaps. The ends play a "four technique," and are each responsible for covering a B and C gap:

The prototypical 3-4 two-gap nose tackle is a 320-plus-pound widebody—a space eater who can take on a double-team and still stop a running back. This player is rarely expected to generate any kind of pass rush. The New England Patriots' Vince Wilfork is a great example.

3-4 defensive ends must be true two-way players, typically 280 pounds or more. Their prime responsibility is to stop the run, but they must also have the ability to beat offensive tackles and sack the quarterback. Few players can truly dominate in both roles, but Justin Smith of the San Francisco 49ers does.

The pass-rushing stars of the 3-4, though, are not the defensive linemen. The size and strength of the men up front allows the linebackers to attack any of the gaps with almost any number and combination of linebackers.

The best pure pass-rushing linebacker typically plays on the right (ROLB), attacking the blind side of the quarterback from the outside edge. The LOLB also brings heat on the strong side, but he may have to fight through a tight end, depending on the protection. DeMarcus Ware of the Dallas Cowboys and LaMarr Woodley of the Pittsburgh Steelers are two of the best 3-4 outside 'backers in the business.

The biggest threat the 3-4 poses to offenses, though, might be the two interior linebackers. Though they can help pick up the coverage slack left by the blitzing outside guys or blitz in combination with them, they can combine to execute one of the most difficult-to-stop blitzes in football: the Double A Gap Blitz.

If you remember the gaps above, you'll immediately understand what this is: the two inside linebackers both blitz up an A gap. Depending on what the nose tackle does and if the tailback is kept in to pass protect, the Double A Gap Blitz often results in a linebacker one-on-one with a tailback, a completely unblocked linebacker or even two unblocked blitzers.

The results can be devastating, as Dom Capers' Green Bay Packers demonstrated against the Patriots. When the nose tackle moves to the outside, drawing the right guard away, the two ILBs cross each other in front of the center, who can only block one of them. ILB Desmond Bishop is left free to deliver the sack.

3-4 Pass Rush Drawbacks

It's well known that a weakness of the 3-4 is the "world theory": There are only so many natural two-gap 3-4 nose tackles in the world, and if you don't have one you can't stop the run. To an extent, it's true of 3-4 DEs as well: not many ends can overpower tackles and tight ends in the run game yet still have the speed and technique needed to sack the quarterback.

Without quality ends, the pass rush must come from the blitz.

Relying on the four linebackers to provide the pass rush is a dangerous game. Linebackers must also share coverage responsibilities, and too-aggressive blitzing can be exploited by good quarterbacks.

Large, strong inside linebackers are crucial, as second-level blockers like tight ends, fullbacks and pulling guards are hard for two-gap linemen to occupy. ILBs must be able to shed these blocks quickly in order to make plays.

The 4-3 Pass Rush

There are several varieties of the 4-3 pass rush, notably the "over" and "under" fronts. Both are traditionally played with a "one gap" philosophy, meaning each lineman is responsible for one running gap/rushing lane.

In the 4-3 Over, the strength of the defensive line is shifted towards the strong side of the offense. The "over" tackle plays between the strong-side guard and tackle, while the "nose" tackle plays the one-technique on the weak side. In this chart, the right (pass-rushing) DE plays a six-technique, but depending on scheme, he could play a seven- or even nine-technique. The strong-side DE plays a seven-technique role just outside the tight end.

The nose tackle—built similarly to a 3-4 NT, like the Baltimore Ravens' Haloti Ngata—will typically draw a double-team from the center and weak-side guard. That leaves the over tackle one-on-one with a guard; as the over tackle is a three-technique DT with great inside pass-rush ability, like the Detroit Lions' Ndamukong Suh, that's dangerous for the offense.

If the defensive tackles are of ideal size and ability, the offensive line is stuck being unable to double-team either without risking the defensive ends coming free. The ideal right (blind-side, pass-rushing) 4-3 defensive end is just big enough to play the run with his hand down, but tremendously explosive and a tenacious pass-rusher. The Minnesota Vikings' Jared Allen is the ideal: a terrifying pass-rusher off the edge with speed and moves to spare, but all the size (6'4", 265 pounds) and strength to stop the run.

The 4-3 Under, on the other hand, puts the strength of the defensive line on the weak side of the offense. The pass-rushing three-technique slots between the weak-side guard and weak-side tackle. The right ("rush") DE is lined up wide as a seven- or nine-technique and is free to pin his ears back and rush the passer with abandon, a la the Indianapolis Colts' Dwight Freeney (though the Colts are transitioning to a 3-4 this summer).

Often, the 4-3 Under is used as the foundation of the "Tampa Two," a one-gap 4-3 defensive scheme where each of the front seven is solely responsible for a single run/rush gap (D/C/B/A/A/B/C). This puts a heavy burden on the weak-side linebacker, who must stop the run behind the pass-rushing RDE and cover a large zone in front of the deep-dropping MLB. It's partially because of the ability of former Tampa Bay Buccaneers All-Pro LB Derrick Brooks that the Tampa Two was created.

In either alignment, the 4-3 defense is ideally suited to generate pass rush from the front four with minimal blitzing. While the 3-4 allows for great complexity, confounding offenses with disguised blitzes and coverages, the 4-3 allows a defense to drop seven men into coverage and still get pressure—an unbeatable combination.

4-3 Pass Rush Drawbacks

Whether Over or Under, one-gap or two-, the 4-3 rush defensive end is a premium position. The Buffalo Bills just signed Mario Williams to an astounding six-year contract with $50 million of guaranteed money because he has the vanishingly rare combination of size, speed and skill to get double-digit sacks without allowing double-digit yardage every time the offense runs at him.

In a one-gap 4-3, maintaining gap discipline is extremely important. There is less flexibility in how you attack the quarterback, because overcommitting or excessive misdirection allows the offense to capitalize by running into the empty spaces. Suh and the Lions were repeatedly victimized by opponents running through the gap Suh abandoned while trying to get upfield.

4-3 linebackers are easy to come by, but outstanding ones are not. The pure run-stuffing MLB who can clean up any gap and make the tackle is a vanishing breed, as the players with the size, strength and speed to play it in college are often moved to OLB or DE and made to blitz.

Also, 4-3 outside linebackers have a hard time making an impact; strong-siders often spend most of their reps shadowing the TE, and weak-siders are increasingly left on the bench in favor of a third cornerback.

The Cutting Edge

The Lions and Philadelphia Eagles play what's commonly called a "Wide Nine" alignment, where the RDE in a 4-3 Over plays from an extremely wide nine-technique position; this stretches the offensive line. Combine great size and depth up front with a get-upfield-at-all-costs approach, and the defense gets maximum pressure with maximum coverage (and minimum blitzing). Unfortunately, the "wide nine" also exacerbates the 4-3's weakness in stopping the run.

Teams like the Patriots have been begun eschewing strict 3-4 and 4-3 labels, playing a hybrid, or "Amoeba," defense. In these defenses, versatile players may put a hand down or play standing up in almost any combination imaginable.

The Patriots will even occasionally forgo linemen entirely and run an apparent 0-6-5. This takes the confusion and the misdirection of the 3-4 to the next level, but it also requires extreme intelligence and versatility from its defenders (and coaches!).

As these new-wave defenses continue their innovative assaults on the quarterbacks of the NFL, it'll be up to offensive coordinators to adapt and adjust. Perhaps we'll see a return to old-school power running to victimize the aggressive front fours of the modern 4-3 and overwhelm the misdirection and masterminding of the modern 3-4.