On first and second down against run-heavy formations, the 49ers run a 3-4 defense, right? Well, not always...

Most of us have at least heard the terms "3-4", "4-3", and maybe even "Nickel" at one point or another. Many of us have a reasonable understanding of what these terms mean, even in their most basic form. We know that the first number refers to how many defensive linemen there are, and the second is how many linebackers are on the field. Sometimes this gets confusing though when you see guys line up in different positions or have different responsibilities.

In this post I'll do my best to give both a basic view as well as some more detail, so you can take as much information from it as you like. For some fans, the X's and O's of football don't really matter much, as long as the offense moves the ball, the defense stops the other team, and our team wins the game. Then there are the rest of us, who love the more intricate details and the inner-workings of the scheme, just as much as we love the highlights of scoring, turnovers and big hits.

For several years now we've all understood and agreed that the 49ers run a 3-4 defense in their "base" package. By base, we generally mean on first and second down, versus a probable run-play. In the Greg Manusky days it was Isaac Sopoaga, Aubrayo Franklin and Justin Smith up front, with Manny Lawson, Takeo Spikes, Patrick Willis and Parys Haralson as the linebackers. Franklin lined up over the center, engaging him while holding his ground and being ready to "shed" to make a play on either side of him. Justin Smith and Isaac Sopoaga lined up over the tackles and generally did the same thing: engage, hold, shed and make a play on either side.

Below is a diagram showing the alignment of the defensive line in a traditional 3-4, with the DEs playing a 5-technique, and the NT playing the 0-technique, as mentioned.





This is in-fact a "true" 3-4 defense, known as a 2-gap scheme. It's called that because of the aforementioned responsibility of the defensive linemen to make plays on either side of them, in other words, in two gaps. More recently however, the 49ers front-seven has looked slightly different than this, specifically since Vic Fangio came aboard.

You'll notice that the nose tackle doesn't line up directly over the center most of the time, and the defensive ends (currently Justin Smith and Ray McDonald) aren't on top of the tackles as they were in the past. You might also have noticed that Aldon Smith often lines up on the line of scrimmage, sometimes in a three-point stance with his hand in the dirt.

Below is an All-22 endzone shot from the Super Bowl showing the 49ers lined up in an alignment that differs from the above image. You'll notice that Sopoaga is shading the center's right shoulder, McDonald is outside of the right tackle, and Justin Smith is between the guard and tackle.

So what sort of defense is this? Brooks and Aldon Smith are both up on the line of scrimmage, looking poised to come forward. It's almost as if there are five men on the defensive line instead of three linemen and four linebackers.

What you see here is actually a 4-3 under front. The term "under" means that a defensive lineman is "covering up" the weak-side guard. "Over" would mean the strong-side guard is covered up. The strong-side is generally where the tight end lines up, or the side where the offensive formation has the most players. In any case, the under/over fronts differ from the 3-4 defense because they don't have defensive linemen directly over the tackles and center, as the first illustration showed.

Now, why is it suddenly a 4-3, even though the same three defensive linemen are on the field as they would be in a true 3-4? The reason is that the weak-side linebacker isn't really playing like a linebacker; he's more of a defensive end. Rather than being back off the line of scrimmage, roaming around waiting for the play to come into his area, he's going forward, trying to get into the backfield and either trip up the running back or sack the QB if it's a pass play.

In this case the weak-side defensive end is Aldon Smith. Now, Ahmad Brooks is also relatively close to the line as well. His job is to read the play and either cover the TE man-to-man if it's a pass, drop into a zone (if it's zone coverage instead of man), or read a run play and contain the edge or make a play. Because of his added responsibilities in coverage, he plays more like a traditional linebacker and thus isn't considered a defensive lineman.

The image isn't the best because it's a two TE set, making it a relatively balanced formation, but the 49ers obviously want to believe this is going to the right side. I say this because if the formation is strong to the offense's left, they'll shift the defensive alignment, like below.

Here you see that the NT is shaded on the center's left shoulder, Justin Smith is outside of the tackle (instead of in the gap between guard and tackle), and McDonald is in the G/T gap. This mean's Aldon Smith has the outside linebacker responsibility of potentially covering the TE or dropping into zone, whereas Ahmad Brooks is the rush defensive end, charged with getting into the backfield and blowing up the play. Again, they flip the alignment based on the offensive "strength" side.

The beauty of the 4-3 under is that you can play it with essentially the same type of athletes as the traditional 3-4, to an extent. What we haven't touched on is the fact that while the true 3-4 uses 2-gap defensive linemen (as mentioned earlier in the post, making plays on either side of their man), the 4-3 under is a 1-gap principle. This means that each player is responsible for penetrating the gap in front of him, not standing and holding the point. The inside linebackers will handle anything that comes through the open gaps.

As I said, you can essentially play the under front with 3-4 personnel, but they must be guys who are capable of penetrating. The idea is that if they are a threat to penetrate the gap, the offense double teams them, taking a blocker out of the equation ... a blocker who could have otherwise released and tied up the linebackers. To a degree, a strong player who isn't the quickest penetrator can still play in a 1-gap system as long as he uses his strength to occupy two blockers. If he can't do that the offense will simply send the adjacent lineman off to clean up the linebacker, and now the running back is seven yards down the field, untouched.

The basic responsibilities of the 4-3 under players are as follows:

The most versatile player is the SAM (strong-side linebacker). He lines up on the outside shoulder of the TE, jams him at the line so he can't get a free release into the pass pattern, must drop underneath in zone coverage or cover man-to-man at times. Oh, and he must also be ready to rush the passer at any time if the call dictates it.

Inside of the SAM is the strong-side DE. This is a run-stopping, large man who's strong and hard to overpower. His job is to stop the run to his side at all costs. I'd say many traditional 5-technique DEs in the 3-4 can play this position, as well.

The nose tackle must be a large man who can withstand constant double-teams from the center and guard. Many 0-tech NTs in the 3-4 can also handle playing the 1-tech in an under front, but not all of them.

The weak-side defensive tackle basically needs to be Justin Smith. An unstoppable force who get's into the backfield no matter what you do to stop him. He needs to be the best interior pass-rusher you have, capable of penetrating easily against 1-on-1 matchups with the guard. Many 3-tech DTs in the 4-3 can play this position, but not many 3-4 5-tech DEs. Justin Smith is not human, however.

Finally you have the weak-side DE; in the 49ers' case, Aldon Smith. This needs to be your best edge-rusher, a beast 1-on-1 vs. left tackles and also a guy who can keep contain on the edge against the run. This is your double-digit sack man. Many pass-rushing 3-4 OLBs and 4-3 DEs can play this position, but they must be nearly unstoppable players.

It should also be noted that I did see the 49ers in a traditional 3-4 alignment as recently as the Super Bowl. It seems that Fangio likes to mix things up, which is why he favors versatile players on his defense. At this point in the NFL though, if you play DL you better be able to do it all if you want to have a long career. Few teams lack creativity and systems change with new coaching staffs and new teams, etc.

The good news is that the lines are somewhat blurred between players who only fit in a 3-4 or 4-3, especially if you have a rotation of players going into the game. An excellent 4-3 defensive end can play for a team like the 49ers now as part of a platoon along the defensive line since he would be great in their under front on the weak-side. All he has to do is line up on the weak-side and do what he's always done: get upfield and hit either the RB or QB while containing the edge.

Granted, having two guys who can play either side of the front is even better. The 49ers are doing this to a degree but it's easier for them because they don't ask much of their edge players in coverage. Still, a spot along the defense can be found for guys, no matter their skill set ... but it's the versatile players who stay on the field the most.

In summary, you'll see the 49ers run a mix of 3-4 and 4-3 under fronts based on the team, formation and specific offensive players on the field. It's just another way that the coaching staff modifies their approach to be as effective as they can be on a play-by-play basis.