Last week we took our first steps into the defensive side of the ball in our Summer School series by seeking to understand the 4-3 defense. As the base defense of the New York Giants, and one that was innovated by the Giants, it made sense to start there.

However, there is another defensive front commonly used in the NFL, the 3-4.

As you may expect, the 3-4 defense is named similarly to the 4-3 defense, though in this case it has three down linemen with four linebackers behind them. The two defenses, however, have very different origins and is better viewed as a case of convergent evolution.

You may recall from last week how Steve Owens, and later Tom Landry, modified the 6-man fronts common in NFL defenses in the 1950s into the base 4-3 defense. The 3-4 defense, however, was first conceived -- that we know of -- at the University of Oklahoma in the 1940s. Because of the demands of dealing with bigger, more powerful rushing attacks in the NFL, pro defenses stuck with heavy lines through the 1960's, but with the growth of offenses in the '70s, the 3-4 front started to make its way into the NFL.

At first, just a few teams were using the three man front, and not all for the same reasons.

Don Shula adopted the three man front when injuries depleted his more traditional four man line. When he found that it allowed him to disguise his coverage schemes, it ultimately lead to the "No Name Defense" that helped the Dolphins become the only undefeated champion in NFL history.

Up in New England, the Patriots installed a more conservative 3-4 defense aimed at containing offenses.

Bum Phillips, father to Denver Broncos defensive coordinator Wade Phillips, took the opposite approach with the Houston Oilers and installed an aggressive, attacking version of the 3-4. It's a defensive philosophy with which his son -- for all intents and purposes Wade Phillips and Von Miller won that game, sorry Peyton -- won Super Bowl 50.

The "Parcells" Two Gap 3-4

When I started our look at the nuts and bolts of the defensive side of football, I decided to go with the 4-3 defense because it's familiar to most current Giants fans. We've been watching that defensive front for a long time, it's a good touchstone. But to take a freshly printed page out of Coach McAdoo's book, I'm referring to the two main versions of the 3-4 in a way to make it more familiar to fans who might specialize in their team.

Fans who go back to the 1980s should immediately recognize the two-gap 3-4 defense as the one Bill Parcells installed when he arrived as head coach, and brought with him to every other coaching stop.

As you can plainly see, the 3-4 only has three down linemen on the line of scrimmage (back slightly here for illustrative purposes). The job of those defensive linemen is what separates the 2-gap 3-4 from the 4-3 or the 1-gap 3-4 fronts.

In the 2-gap 3-4, each of the defensive linemen is responsible for two "gaps", designated by the letters between the offensive linemen. In the 4-3 defense, only the nose tackle is responsible for two gaps, and even then that depends on the defensive call.

Because of their different responsibilities, the defensive linemen in a 2-gap 3-4 defense tend to be much bigger and stronger than in the 4-3 or even 1-gap 3-4 defense. A 2-gapping defensive lineman has to be powerful enough to take on a double-team at any time, or control an offensive lineman with one hand and make a tackle with the other. Translating this into the Giants' personnel, the right defensive end would be Jay Bromley, the nose tackle would be Damon Harrison, and Johnathan Hankins would be the left defensive end.

What the 2-gap 3-4 lacks in opportunities for the defensive line, it more than makes up for with the linebackers. This is the defense that allowed Parcells to fully unleash Lawrence Taylor upon the NFL, forever changing the way the game is played.

With just three lineman occupying up to six blockers, it allows defenses to disguise their coverage schemes and send pressure from anywhere on the field. For Parcells, that meant moving LT all over the defensive formation, forcing offenses to constantly find and account for him.

In the modern NFL the 2-gap 3-4 is rarely used as a base defense, however it is carried on by Dick LeBeau (formerly of the Steelers, and currently of the Titans) and Dom Capers (of the Green Bay Packers). These two defensive coordinators use the 2-gap 3-4 front to call a variety of zone blitzes (a lesson for a later date), confusing and creating havoc for offenses.

It is, however, difficult to play a true 2-gap 3-4 front in today's NFL. Players like Vince Wilfork, Haloti Ngata, or Damon Harrison are rare, and there simply aren't enough good, big, linemen to go around.

The "Phillips" One Gap 3-4

As the name suggests, the 1-gap 3-4 defense sees the majority of the defensive front attacking a single gap. This version of the defense, made popular by Bum Phillips, is a much more aggressive version of the 3-4, sending the defensive linemen into the backfield as opposed to just holding blocks.

This is also where the lines between the 4-3 and 3-4 defenses start to blur.

Seeing it diagrammed out, it's easy to see the influence of the 4-3 front on the 1-gap 3-4.

First, the whole defensive line is in an "Under" shift, shading away from the strength of the offense. Like the 4-3 Under, this gives them the ability to apply pressure and be disruptive while creating a "numbers game" for the offense. And because of this defense's focus on attacking with the defensive line, it allows for smaller, more athletic defensive ends, such as former San Francisco 49er Justin Smith or Houston Texans star J.J. Watt. Of course, the term "smaller" is relative here, but 290 pounds is less than is required by a 2-gapping defensive end.

For the Giants, the defensive line would would likely be manned (from right to left) by Owamagbe Odighizuwa at right defensive end, Johnathan Hankins at nose tackle, and Jay Bromley at left defensive end.

The linebackers also get different names in the 1-gap 3-4, introducing the "JACK" position. It has also been called the "LEO", "JOKER", or "ELEPHANT" linebacker, depending on the defensive coordinator and their specific system, but to avoid confusion we'll just go with "JACK". The "JACK" position is usually a player that is referred to as a "Tweener" coming out of college, or a player who is too big to be a traditional linebacker, but too small to routinely take on offensive linemen. In many ways, the JACK is similar to the weak-side defensive end in a 4-3 Under defense. Though, a smaller more agile player could be preferred, giving him greater ability to drop into coverage than a traditional defensive end and create confusion for the offense. For the Giants, both Jason Pierre-Paul and Olivier Vernon could be effective JACK linebackers.

The strong outside linebacker or SAM, is similar to the SAM linebacker in 4-3 defenses. He must be powerful enough to take on a tight end in the run game, quick enough to be an effective pass rusher, and agile enough to cover a running back or tight end in pass coverage. Once again, Devon Kennard is almost exactly the prototype for this position.

The many similarities between the 4-3 Under and the 1-gap 3-4 defensive fronts are what allow Steve Spagnuolo to call a "multiple" front that freely switches between these two looks.

But what happens if a defensive mind takes the next logical step and calls both at the same time?

Hybrid Defenses

There are almost as many ways to create or call a "Hybrid Defense" as there are defensive minds in the NFL. Is the defensive coach coming from a 3-4 background or a 4-3 background? What does the team's personnel dictate?

Almost every NFL defense is "multiple" and uses a variety of looks and packages to fit the situation. There are also several "hybrid" defenses that incorporate principles from both the 3-4 and 4-3 fronts. However, when the words "Hybrid Defense" appear, it is Pete Carroll's defense in Seattle that immediately leaps to mind.

The Seattle Seahawks' defense wasn't really revolutionary in its ideas, at least not the same way as Landry's 4-3 defense, or Dick LeBeau's Zone Blitz concepts. Carroll's innovation was to use what is effectively 3-4 personnel to fill out a 4-3 defensive front, and put them in position to make it work.

The alignment is obviously a 4-3 Under front -- the Seahawks have used an Over front as well, with the defensive tackle on the "strong" side of the offense -- but looking at the assignments, this is two different defenses.

The right half of the front is playing a classic 4-3 Under defense. The "LEO" takes the place of the weakside defensive end, but could also be called the "JACK" from a 1-gap 3-4 defense. The "DT" is the same as the Under Tackle from the 4-3 Under defense we looked at last week, and both players attack a single gap.

On the left side of the line, the NT is a classic 3-4 nose tackle, with the same job as the defensive fronts we discussed above. His job is to control the center of the offensive line, take on double teams, and be an immovable object that can make a tackle if a team runs at him. The left, "base", or "closed" defensive end plays two gaps as well, and is similar to a defensive end from a 3-4 defense.

Translated to the Giants' defense, Both Olivier Vernon or Jason Pierre-Paul could play the LEO position, though neither really has the explosive quickness that players like Bruce Irvin, Von Miller, or Khalil Mack possess. On the other side, Owa Odighizuwa or Kerry Wynn are good fits as the left defensive end, with the strength to control both the right tackle and tight end if necessary.

Both Harrison or Hankins can play the nose tackle, while Hankins and Jay Bromley can play the Under Tackle.

Final Thoughts

Like last week's look at the 4-3 defense, this was just an introduction to the schemes and concepts of the 3-4 defense. Much like the 3-4 defense, there have been many takes on the 3-4, tailored to fit teams or even players with special talents.

One thing to keep in mind when thinking about defenses in the NFL is that while these concepts are the starting point, they aren't the ending point. Few, if any, NFL teams are all 4-3 Over, or 2-Gap 3-4, or run any one front. They all mix and match concepts, looks, and packages, and none of them really fit into neat little boxes.

Does that mean that we've wasted our time the last two weeks? Of course not! Recognizing the basics of all the schemes and alignments gives insight into what the defensive coordinator is trying to do on any one play, or recognizing what he wants to do from the base defense -- which is really the defense they'd like to stay in for as many plays as possible. It just means that football is more complicated than two teams of very large men banging into each other for an hour while they each try to move an oddly shaped ball to one end of the field or the other.

Next week we'll put a bow on the defensive line and start our discussion of the defensive secondary.