Buffalo Bills depth chart 2013: defense scheme, alignment

I had an interesting question pop up in my email inbox this morning that I thought I'd whip a quick post up on over my lunch break at the day job. The question, from a reader named William, reads as follows: "Brian, could you explain the positions used (and the players at them) in your Bills depth chart? 'RU' is especially confusing to me."

William asks a legitimate question that deserves a reply that's in-depth enough to be posted here and linked to him via email. Here's the specific part of the depth chart that William is referencing:

Then there's the below end zone still of the Baltimore defense, which provides the template on which I built the depth chart, for reasons I'll explain below:

From left to right across Baltimore's defensive line, the positions translate to my depth chart respectively: SLB, DE, NT, DT, RU. At the linebacker level, left to right, it goes MLB, WLB (though that's based on the defense's alignment, and not so much what the offense is doing).

• SLB is the strong-side (often referred to as "Sam") linebacker. He lines up on the strong side of the formation, over the tight end, and can either drop into coverage or rush the passer out of this alignment (the 4-3 Under). Lawson was signed as a free agent for this specific role. To the naked eye, yes, this position looks like a 3-4 outside linebacker role, but it's functionally closer to a 4-3 linebacker role with a rush element mixed in.
• DE obviously stands for defensive end. This player can line up in a five (head-up on the tackle) or six-technique, and functions like a defensive tackle, controlling a gap (or two) in the middle of the formation. I put Branch here because of his ability to play one or two gaps; Dareus can also play this role. This most closely mimics a 3-4 defensive end role, but is not strictly a two-gap position.
• NT is the nose tackle. In this defense, that role is a one-gap role from a one-technique (off the outside shoulder of the center) alignment. Dareus and Kyle Williams can both play this role; Kyle might be the better fit for it, actually, but he's currently better than Dareus is at the next position on our list. The Bills might be best-served to get a nose tackle that can two-gap down the road so that they're not always playing in an Under front; Branch might give them the best option to do that at the moment, and he's not a snug fit for that role.
• DT stands for defensive tackle, and for our purposes it means a three-technique (off the outside shoulder of the guard) player. Ideally, this is where your top interior pass rusher plays most often, which is why I have Kyle Williams lined up there - but obviously, Dareus can play here too (as can Branch, for that matter). Carrington is also a capable sub-package and rotational player in this role. This is the position at which your best defensive lineman, period, usually lines up.
• RU is the two-letter shorthand I use for the designated pass-rusher role. Mario Williams is this team's designated pass rusher, and Anderson is his reserve because he's the team's situational pass rusher. Mario can play other roles (DT and elements of the DE specifically), but he'll spend most of his time playing run contain and getting upfield. I'd listen to arguments that Anderson could play a little SLB, but really he's just a straight up pass rusher.
• MLB (middle or "Mike" linebacker) and WLB (weak-side or "Will" linebacker) are common designations, and their alignment is based on the strength of the offensive formation. Ideally you have two players that can play both roles and be interchangeable, but you also typically want your best play-making linebacker to be on the weak side.

Buffalo's got some versatility in its front seven - particularly between the defensive line trio of Williams, Dareus and Branch - but more is needed if they're going to easily flip between even and odd fronts, and between one- and two-gap schemes. Right now, I've aligned the unit based on two factors: getting the best players on the field, and playing to those players' strengths. I hope this helps, William!