(Written by kencraw)

Cal continues to use a lot of the 3-4 in spring practice. This has been rumored change for 2008 over the off-season with the struggles the defensive line had in 2007 and the wealth of talent the Bears have at linebacker. An important caveat though was that there was similar talk last year after the departure of Brandon Mebane and despite that talk, the 4-3 was used almost exclusively during the season. Add in the defensive coordinator Bob Gregory has said to not read much into the majority use of the 3-4 and you should take these rumors with more than a grain of salt.

But that won’t stop me from blogging about it…

Here is my best shot at giving a primer on the 3-4 and how it is different than the 4-3.

The biggest implication of the 4-3 vs. the 3-4 is the “gaps” that are filled and by who. The gaps are the spaces in between the offensive linemen. These gaps are the two-way street that either the running backs are trying to get through on running plays or the defense is trying to get through on their way to try and sack the quarterback on passing plays. The gaps are labeled as follows:

• The “A” gap is the space between the center and the guards. Thus there are two A gaps, one on either side of the center.
• The gaps between the guards and the tackles are called the “B” gaps. There are of course two of those as well.
• Finally, in some circles, the space outside the tackles is called the “C” gap. This doesn’t make much sense if there is no tight-end as the “gap” is all the way from the offensive line to the wide receiver. However if there is a tight-end, the C gap is a normal gap just like the others.

With a 4-3, all of the A and B gaps are covered by the 4 defensive linemen. That leaves only the C gaps to be covered by the linebackers which is generally easy to do because most teams don’t use a double tight-end formation often so only one linebacker is consumed by that. That side, called the strong side, often is compensated for by bringing in a strong safety to help with the tight-end who can either be a blocker on a run play or a reciever on a pass play. There are more scenarios here than anyone could cover but the key piece of information to remember is that the A and B gaps are covered by the 4 defensive linemen, with each defender assigned to a single gap.

What that means is the the bulk of the offensive line is taken up handling the defensive line. If they put one lineman on each of the defensive line, that leaves just one lineman for either stunting (where they leave their position and move to another spot for extra blocking support) or double-teaming a particularly strong defensive lineman. Of course more complex blocking formations exist where a lineman stunts even when it means leaving a defensive lineman unblocked (to be either picked up by a another player such as a fullback or to be run away from so that it doesn’t matter that he’s not blocked) but for your base formations, the key point is that 4 defensive linemen pretty much occupy all the major gaps and therefore occupy most of the offensive line’s blocking ability. This leaves the linebackers free to run to the ball on running plays, or either blitz or fall back into coverage on passing plays.

Hopefully this illustrates the balance of the 4-3. It puts both sides of the ball on pretty similar footing. Since a stalemate is a good thing for the defense, particularly on running plays, one can see why this is a popular defensive scheme. However, when the 4 defensive linemen are under-performing, it also gives an advantage to the offense, particularly on passing plays.

The standard way a 5-man offensive line blocks a 4-3 defense on pass plays is to have the center and guards collapse the A gaps. That takes care of the defensive tackles who generally are trying to get through the A gaps. That leaves the defensive ends working against the offensive tackles who are either trying to get through the B gap or spin outside to the “C gap” if there is no tight-end blocking. Generally speaking the defensive ends can either get around the ends quick enough or try to “shoot” the B gap requiring assistance from the guard and freeing one of the defensive tackles, so that the 4 defensive linemen can get pressure on the QB without assistance from the linebackers. Additionally, the 5 linemen have their fill with the 4 defensive linemen so any linebacker blitz is going to have to be picked up by a one of the skill players in the backfield (tight-end, fullback, runningback) or the QB is going to be in trouble quickly. However, with a weak set of defensive linemen since the offensive line has each man covered, they won’t be getting much pressure and in this scenario, the stalemate works in favor of the offense.

All of this brings us finally to the 3-4.

The 3-4 is a weaker run defense when one ignores the talent level of the players involved. With 3 defensive linemen to cover 4 gaps (both A’s and both B’s) it requires that the defensive linemen well anticipate which gap the running back will be going towards. The nose-guard is assigned both A gaps and the defensive ends cover their side’s B and C gaps. The linemen need to anticipate when gap the offensive line is trying to open up and choose the appropriate side of their offensive lineman to fight for. This also requires that the linebackers be well in sync with their linemen to cover their holes. Even when they are in sync, the extra 3-5 yards off the line the linebackers are makes for a difficult run stopping situation.

The pass-rush situation, on the other hand, isn’t nearly so bleak. Now that it is the offensive line who has to guess which gap to fill (because the defensive linemen have two gaps they can go to). The standard A gap collapse that the guards and center like to do doesn’t work as it’s applying 3 offensive linemen to a single defensive lineman. Also, the offensive linemen know that they’re going to get at least one linebacker rushing and it’s again a guessing game for them to figure out which gap is going to be overloaded. In many ways, the 3-4 gives the defense a lot more flexibility for passing downs. They can drop 8 into coverage if they desire with 4 linebackers or they can easily rush 5, with some flexibility as to which 5 that might be, while still leaving 6 in coverage.

Looking back on 2007, one can see why it might be tempting to go for the 3-4 in 2008. The rush defense of the Bears was fairly strong, but their pass rush was one of the weakest in the conference. The 3-4 optimizes towards making it easier to pass rush while increasing the difficulty of the run stopping. Assuming our talent levels stay the same, this would balance out the strengths/weaknesses of our defense, or at least that’s the theory.

Will the run stopping penalty outweigh the increased pass rush? Only time will tell. Nevertheless, I hope this primer is helpful for those who may not know much about the difference and what to look for if the Bears start to use the 3-4 more next season.