The 3-4 Defense can be a pretty non-descript term. There are a significant number of variations on the basic concept of 3 down linemen and 4 linebackers.
While you see some variation in the 3-3 Stack, 4-3 Defense and 4-2-5 Defense, the variations are fairly limited. For example, in the 4-3 Defense you may see the Strong Defensive End in a 6, 7 or 9 technique on the Tight End. Linebackers may move a half man one direction or another. But all in all, you will recognize it as a 4-3 Defense.
While you may recognize some varieties of the 3-4 Defense immediately, other coaches who refer to their system as a 3-4 Defense may look more like a 4-3 or 4-2-5 Defense on any given down.
The True 3-4 Defense
Charlie Weis has said that the true 3-4 Defense is the Fairbanks-Bullough 3-4 Defense, which was used by Bill Parcells and the New York Giants in the 1980s. This version of the 3-4 Defense may not have been the first, but it is what most people think of when they think 3-4 Defense.
The Fairbanks-Bullough 3-4 featuers 3 defensive linemen. The Tackles are head up on the Guards in 4-techniques, and the Nose Guard is in a head-up 0 Technique on the Center. Two Linebackers are in 30-Techniques over the Guards, and two more Linebackers are outside, somewhere between 1×1 and 3×3 off of the End Man on the Line of Scrimmage (which can vary much more depending on the offensive formation).
The Defensive Linemen are playing a 2-gap technique, attacking and reading the Defensive Linemen. One of the Linebackers will be included in the pass rush in most cases. This version of the 3-4 Defense matches up well with Cover 2 and Quarters coverages because of its 7 man front.
While the Giants may have had a great deal of success under Parcells, most teams today do not use this version of the 3-4 Defense.
The Slant & Angle 3-4 Defense
The next variation of the 3-4 Defense uses the same initial alignment as the Fairbanks-Bullough 3-4, but a more aggressive slanting attack by the defensive line.
With the slanting of the defense, the Defensive Linemen become responsible for only one gap. They are able to play faster and cause havoc for the Offensive Line. Much like the 3-3-5 Defense, the slanting used in this version of the 3-4 Defense means the Offensive Line has no idea where the defenders will end up being responsible for.
This version of the defense will still involve one of the Linebackers becoming involved in the pass rush, usually the Outside Linebacker to the back of the slant.
Again, the coverage is very flexible with the 3-4 Defense, and you can see this front matched to a number of different coverage schemes.
The Multiple Front 3-4 Defense
A very popular version of the 3-4 Defense today is the Multiple Front 3-4 Defense, which often does not look like the traditional 3-4 Defense fronts. In fact, the alignment is often more reminiscent of a 4-3 Defense. This version of the 3-4 is often referred to as a “Hybrid 3-4 Defense.”
The front remain a 3-4 Defense, because three of the defenders are considered Defensive Linemen 100% of the time. A fourth defender will usually be involved in the pass rush, but will have some drop responsibilities while another player takes his place in the pass rush.
The alignment of the defense is usually set to the Field or Boundary, to get the best defensive match-ups. While you can still use the 4-3 Defense principles of bending and spilling (harder when you are slanting), your fourth defensive lineman remains a mystery to the Offense until the play starts.
You can still utilize the head-up alignment and slanting of the traditional 1-gap 3-4 Defense. You can also use a variety of fronts like the Over, Under and Bear Fronts to add more versatility to your defense.
With the wide variety of options, you give yourself limitless freedom with coverages. This variety of the 3-4 Defense is also excellent for setting up great blitz angles for both Man Blitzes and an extensive Zone Blitz package.