Other base fronts
The Bills played some snaps in what looked like a true 3-4 defense. Typically, defenses that play out of "true" 3-4 fronts are purely 2-gapping defenses. However, every time I saw the Bills line up in a 3-4 they played one-gap principles (at least with their ends) and sent both DEs crashing into the B gaps. Below, you'll see one of those times.
The Bills lined up in the traditional 4-tech/0-tech/4-tech line of 3-4 defenses, with ILBs at the 20 techniques, directly over the offensive guards but off the line of scrimmage. The outside linebackers were at 6 or 9 techniques, outside or over the TEs.
On the snap, you'll see that the Bills are not playing a traditional 3-4, as the ends stunt into the B gaps.
Typically, the even techniques signify a 2-gapping player, but it did not appear this was the case for Mike Pettine's "3-4" front in Buffalo. Despite being at 4-techniques, those DEs are clearly one-gapping players.
As a side note: to a coach an "even front" doesn't mean a front with an even number of players (e.g. four) in it. An "even front" is actually one where the defensive linemen line up in even techniques, like this front. Likewise, an "odd" front isn't one in which there are 3 down linemen, but one where the linemen line up at odd techniques.
Pettine almost assuredly learned something about this front from Rex Ryan, whose father Buddy Ryan invented it. Sometimes called a "Double Eagle" front with two DL aligned at 3 techniques and a third inside of them at a 0 or 1 technique, teams employ the Bear front to put a ton of pressure on offensive blocking schemes.
The original Bear front lined a nose tackle directly over the center and two defensive tackles directly over the guards. Pettine (and most coaches in 2014) spread those tackles just slightly out to 3 techniques on the outside shoulder of the guards. The Nose tackle will play at either a 0 or 1 technique. One of those "tackles" would be a "defensive end," like Desmond Bryant or Athyba Rubin. We're still talking about 3 x 300+lbs players, with the NT likely being closer to 350.
Outside of those three big linemen on one side of the line is a "defensive end" who would be someone like Sheard, Kruger, Mario Williams, or Jerry Hughes. Here, Williams is lined up wide in a ghost 9 technique. Opposite him and over the tight end are both "outside linebackers," (if you look at Pettine's base defense as a 4-3). For the Bills, those were Manny Lawson and Arthur Moats.
Behind the line, the last (Mike) linebacker typically lines up on the strong side, while the defense brings a safety down into the box to play over the weak side of the line. These players will be the ones flowing to the ball in the Bear front.
You can see above that many small adjustments are made to this front to be able to defend the additional TEs and receivers the offenses have placed near the line. But recognizing a Bear front is always easy; just look for those two three-technique tackles with a third nose tackle inside of them.
With so many enormous men over the three interior offensive linemen, the offense has a difficult time achieving double-teams or pulling linemen. If the offense wants to pull a guard using the Power O play, for example, the center must block all the way down on a 3t DT, sometimes needing to cross the face of the NT. This block is extremely difficult to make, if not potential suicide. The offense can also try to have the weakside tackle reach block the DT, turning the Jack loose. This block is pretty damn hard too, though.
The playside guard must then block down on the NT (potentially in the 1-tech away from him) which is also difficult. That leaves the tackle to block down on the DT to his side, which is the easiest of those three blocks.
With 8 in the box, the Bear defense has the numbers to stop the Power O play from traditional pro personnel. And with all of those tough blocks on the backside of the play, all the linebackers on the frontside have to do is stack up their blockers and wait for the backside linemen to win. The SS and Mike can fast-flow to the ball, knowing that the linemen have all the gaps on the backside of the play covered.
Of the games I watched, those were all of the Bills' fronts from base personnel (there could be more out there). It seemed that when they were in base the Bills kept the amount of fronts to a minimum, staying true to the old adage "play defense, not defenses". They varied coverages, but didn't get nearly as complex with blitzes and fronts as they did in sub packages, which I'll cover next.