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An "under"standing of Denver's base defense: Part one

How do we define a base defense? Where do we look for it in games? What field situations, down and distance considerations or personnel permutations are conducive to playing in base? And what can defining a base defense really tell us about the design of the coaches and success and failures of the playmakers?

Kevin C. Cox

As an analyst breaking down film, I always struggled with these constraints for "defining" a team's defense. Popular analysis by fans, ex-players and network pundits would only temporarily inform my film-watching, more often bogging down under a burden of bias when what I watched on a play to play basis didn't match up well with what had been described to me. Eventually when applying these observations, I would have more "exceptions" to the rule, than instances of the rule, and few--if any--truly "base" looks from the defense.

And why does identifying the base defense even matter? If you listen to these self-same pundits talk, teams spend more time in sub-packages than in their base anyways. In some games, a team may only run a small handful of plays from their supposed base defense. Furthermore, if you mistakenly identify the base defense, what follows is a procession of round-peg, square hole mistakes indicting players, coaches and schemes, as phenomena such as an alley player's timely awareness get deformed into lamentation's of a player's lack of pursuit or recognition. So why pin ourselves down trying to understand the base defense?

Its a good question, and the answer really depends on how informed you like your opinions--and blame--to be.

Base Defense, Defined

Ask someone to define a team's base defense and you will get a handful of possible standards against which to measure this.

  • Percentage of snaps. The majority of looks are out of base.
  • Field situation. Base defense is a 1st and 10 defense.
  • Stopping the run. The base defense is for run; sub-packages are for pass.

There could be more, but they all share a myopia regarding what actually gets run in games. Majority of snaps? Doesn't fit the numbers, especially the seasonal totals where sub-packages outnumber "base" downs almost 2 to 1. Field situation gets us closer, but not all 1st and 10s are created equal. Some occur with a tremendous lead, others with an insurmountable deficit. Some are early in the game when the teams are still feeling each other out. Others are late in the game , or in the red zone, or part of a two-minute or four-minute offense. Stopping the run is the closest I see in this list to an accurate description of a base defense, but it leaves out an important component of playcalling: almost EVERY defensive call includes a coverage call. Outside of goalline defense, all the defensive backs and at least one linebacker are reading keys that include coverage responsibilities. If these are part of every call, then they are most definitely a part of the base defense, so base isn't purely about stopping the run.

The reason that stopping the run seems close to identifying the nature of a base defense is because stopping the run is considered a football fundamental, and for good reason. An offense's play selection and design criteria is limited only by square yardage of the field of play (6399.6 sq.yds), a number that is limited by the reasonably useful yardage behind an offense (yards required for a dropback, end around, pitch or punt) and the positional limitations as players are moved farther out from the ball towards the sideline. Within that space teams can run, pass and kick, and in order for a defense to successfully take downs or possessions from an offense, they have to constrain those choices. Thus the time worn mantra of stopping the run. Running threats are naturally limited to near the line of scrimmage, and thus they are easier to align against than passes which cover too much ground, too quickly. Once you have successfully established yourself against the run (in some cases purely through the phenomenon of deterrence, such as with a 9-man front), the passing plays become significantly constrained. And once you have a down or two of success against the pass during a series, an offenses choice's become severely much so that the punt from union rules rugby survived into the modern incarnation of the game, as a safe "out" for an offense that has been shut down.

With such fundamental import, stopping the run serves as the beginning of a defensive gameplan, which is why it seems like it could describe the purpose of a base defense, which is the beginning of a defensive scheme. But as we have noted, a base defense must do more than stop the run, and stopping the pass is implied in it as a scheme.

Denver, and the 4-3 "Under" Defense. Oh, and Hybrid Defenses. And Being Multiple.

If you read MHR or follow the Broncos closely, you have probably been thinking, since the start of this article, "Denver runs a 4-3 "Under" defense, problem solved!" And you would be right. Sort of. Denver does indeed run a 4-3 "Under" and they run it a lot.

They also run a 3-4, a 5-4, a 3-3, a 4-2, and a 5-2. A lot. In some cases, especially for the 3-3 and 5-2, they run these far more than the 4-3.

"That's because it is a hybrid defense," you say. "It morphs as easily from a 30 or 50 front to a 40 or 60 front. They are practically the same formation!"

So we aren't a 4-3? Is that what "Under" means? That we are running the 'not-4-3'?

"We are multiple. Get over it. This stuff is meant to confuse the best defensive coordinators in the league. They lose sleep figuring these things out. That it has you chasing your tail just means that it is working."


I'm trying to understand. I really am.

Do we stick to the concretes, call our base the "4-3 Under," and proceed to file all our observations and evaluations under this heading? And when they don't mesh, when we run a 3-4 or two gap a defensive end or tackle, do we file those separately as sub-packages and exceptions to the rules of our base defense? And after we amass literally hundreds of these exceptions, then what? Write a book to contain all that information? Install computers in our brains so that we have a realistic chance of processing all that data?

Or do we file them all under the concept of a hybrid defense, thereby dumping everything we studied and learned with a vacuous concept that describes everything yet means nothing? Knowing your defense is multiple is one thing. Understanding the value (and drawbacks) of multiplicity is another. To fall back on a hybrid or multiple designation in describing a base defense is to throw up your hands and admit that you have no hope of fully understanding the roles and responsibilities of the playmakers we sign and see on Sundays.

I've been to both sides of this divide, of understanding and blame. I've excitedly broke down film with some new "fundamental" observation as my guide, finding small hidden truths in a game of exceptional complexity and duality. And I've been overwhelmed by the details, lost and searching for some clue that our players were improving and getting better, that our team had hope. Analysis serves this purpose if for no other reason than to, as Kipling put it,"...fill the unforgiving minute with sixty seconds of distance run."

So, intending to salve the wounds of 3 years worth of devastating post-season exits (two-years worth actually, as I was inspired to do this after the Baltimore loss of 2012, though it wasn't finished until after the 2013 Super Bowl), I set out to gain a deeper understanding of our defense and its progress, its triumphs and failings.

"When you plant lettuce, if it does not grow well, you

don't blame the lettuce. You look for reasons it is not

doing well. It may need fertilizer, or more water, or

less sun. You never blame the lettuce. Yet if we have

problems with our friends or family, we blame the other

person. But if we know how to take care of them, they will

grow well, like the lettuce. "

Thích Nhất Hạnh

I broke down every defensive snap over the last three years, noting every identifier I could, including 3-4, 4-3, 5-2, odd, even, over, under, cover 3, cover 2, robber, man, zone, nickel, goalline and many more

With no disrespect for others who had set out in search of these definitions before me, and in many places relying on their work, I broke down every defensive snap over the last three years, noting every identifier I could, including 3-4, 4-3, 5-2, odd, even, over, under, cover 3, cover 2, robber, man, zone, nickel, goalline and many more. I had to hit the history books in some cases to identify certain oddities such as the rare cover-9 look (pretty close to what we call a dime, and probably treated as a subset of same by Broncos) or the 5-4. As I parsed these many and disparate classifications, I was able to remove many from considerations of being our "base". Some were used so infrequently as to remove themselves. Unexpectedly, the contradictions that plagued my playmaking analysis (3-4 with a 2 gapper, 4-3 with all one gappers) yielded fruits in this investigation, as having two opposed schools of thought present indicated that while both were adjustments to the base, neither was the "true" base.

Over time I whittled the concepts down, methodically eliminating anything that was nonessential to the base defense, things that weren't inherent in all forms of the defense. At one point I had eliminated so much I felt that I was either missing some critical observation and thus pursuing an exercise in futility, or that what Fox/Del Rio were doing was so advanced that a general understanding of the history of the game wouldn't be enough to ferret out its identity. But finally, in the end, I found myself at the "over-under" dichotomy.

Over and Under describe a defensive line adjustment with the 1- and 3-tech, and that they should emerge somehow as the essential building block of the defense was a little surprising. More surprising was just how little the Over adjustment was used, and how specific the instances for it were, instances where we were protecting large leads early in games against good interior running games (Oakland jumps to mind here). It has been understood for a while now that Denver is an "Under" team, so I'm not bringing this up as a eureka! type moment. Rather, it helps to understand my mindset when I set out to discover whether a simple defensive line adjustment could be something fundamental to a scheme.

The Under as a Base Defense

The first thing to understand about a base defense is that while it needs to be specific, the less specific it is, the more versatile it will be when it comes time to gameplan for any given Sunday. Its framework should be broad enough to support plenty of ideas, old and new, from week to week, but simple enough to be structured and taught to a rookie or free agent.

The key is to strip away specifics that are too limiting, while maintaining specifics that could potentially support a lot of adjustments. I polled the coaching boards at CoachHuey and other places asking if the Under concept could be just such a concept, and I received a resounding "Yes!" in response, with more than one observation that such a defense was probably the most versatile coverage defense you could design. Many coaches were running under front base defenses and were very excited to talk about the whys and whatfors. Intrigued by all this input I started to put together their details into a more comprehensive description, and that is what I would like to share here on MHR, in the following weeks.

To start with, I wanted to know why the Under, and not something else? It all starts with that defensive line adjustment I mentioned before.

In an Under front and in an Over front, you have a nose and a tackle. In the Under the nose lines up in a 1-technique (over the inside foot of the strongside guard). Then you have a tackle line up in a 3-tech (over the outside shoulder of the weakside guard). In an Over front, the opposite shift happens, with the 3 tech on the strongside and 1 tech lined up weakside. From this fundamental center, all the other positions and roles radiate, establishing themselves in order of their distance from the center, with the Free, or free safety being the final piece of the puzzle.


As you can see in the diagram, what this accomplishes is to either shift the strength of the defensive line towards the strength of the running offense (Over), or to shift it away from the running strength (Under). Whether there are 3,4 or 5 defensive linemen is less important than this shift which essentially establishes (or de-establishes) leverage against the run. Note that the non-shifted variations of these fronts puts an equal basis on either side of the line, with the odd player splitting on the center.

So, fundamentally, the Over is the stronger run defense, whereas the Under is merely "gap-sound" (i.e. a player in every gap). The Over's leverage comes primarily from two factors: it creates a mismatch with a lineman covering the tightend (usually), and the center, so the two most compromised blockers on the offensive line are challenged, which can lead to good run stuffing lanes for the linebackers.  Secondly the shift puts more "hand in the dirt" defenders to the strength of the offense.  The Under on the other hand creates an overhang player out of the SAM, which compromises his effectiveness against the run, so in an Under scheme, the SAM comes to the line of scrimmage whenever a TE is present.

So if the Under starts out so bad against the run, why choose it as a base?

Despite weakening the front vs. strongside runs, the overall effect once the SAM comes to the line is to overcompensate on the strong side, while maintaining an adequate run stopping presence on the weakside. Nowhere is this more apparent than in a "bubble analysis" of a few common fronts.


One bubble over the MIKE LB in the Under front. Note that there is some disagreement on whether the weakside A-gap over the Will is a bubble or not. Technically it is not, but a strong case can be made for schemes designed to attack it.


Two bubbles over the MIKE and the WILL, in the 3-4.


3 bubbles over the SAM, MIKE and WILL, in the 4-3 Over front.

Gap control is not unique to the Under of course, but is is necessary for any defense if it is to be called a base defense. And while every player has an assigned gap, movement of the offense can and will change gap assignments, and the Under has a very simple system for handling gap-exchange, making it a great base choice. The gap-exchange simplicity allows new and younger players to execute better, and allows the defense as a whole to move and play faster. Gap exchange is one of the concepts we will cover in depth later.

On the same note as playing fast, the under shifted tackle (called an undertackle in some schemes) is very hard to reach with a double team, giving him a good chance to penetrate and be disruptive. And the rest of the defensive line, even if double teamed or combo blocked, dictates through alignment that the offensive blockers must block and release quickly or risk getting overpowered away from the block. This in turn allows the defensive line to press aggressively and be a more of a factor in defending the run. This then frees the linebackers to play downhill, quickly attacking an open playside gap.  We'll look at linebacker keys and responsibilities in-depth as we go forward with this series.

Normally, the linebackers would have the coverage call weighing on their mind, but in the Under, as long as the playside gap they are attacking is open, they are automatically right, since the coverage players behind them are reading the backers and will be the ones to adjust their support assignment. We will cover this later, when we talk about specific coverage meshes in the Under.

Of particular interest, is a unique side effect of the alignment of the two outside shade players in the base Under. This alignment creates a 'tandem' concept on the ends of the line. Against teams that attempt to run outside or execute kick-out blocks on the ends, these tandems prove tremendously advantageous. An offense would be required to execute two reach blocks in a row to pull off an outside zone run or toss-sweep. To run counter, lead or power, they would have to execute a double team and quick kick-out block, all while another defender is in close proximity. Few if any T/G combos in the league are consistently quick enough to pull that off, and in the meantime one of the defenders is getting through to the backfield for the stop.


But this just barely scratches the surface of the Under's capabilities. What we see here is that the Under alone can be a flexible, multiple, fast playing concept upon which innumerable concepts can be stacked. I hope I have shown that there is really no need to burden the understanding with a qualifying designation of "4-3" or "odd" or "even". And it is my hope that as we learn of the roles, assignments and alignments of the playmakers in the Under, that we will have little cause to search outside of its framework for a foundation upon which to observe and understand what Denver is doing on a week to week basis during the season.

I look forward to continuing this series as we gear up for the 2014 season!


I am including this postscript to provide some basic terminology and information that will be helpful as we move forward in this study of the Under defense.

  • The first is BroncoMike's MHR: University article featuring a breakdown of lineman gaps and techniques. You'll need this info to understand a lot of what we are going to talk about.


  • Next is a description of the roles and keys of the 11 base defenders:


S=Sam: an outside linebacker who keys on the tightend

E=End: the defensive end who aligns with the Sam

N=Nose: the defensive tackle that aligns to the Sam

T=Tackle: the defensive tackle lined up away from the Sam

B=Bandit: Defensive end lined up away from the Sam

M=Mike: An inside linebacker lined up on the Sam’s side

W=Will: An inside linebacker lined up away from the Sam

R=Rover: A defensive back who is ALWAYS lined up away from the Sam

F=Free: A defensive back who is not tied to the front, thus ‘free’ to move at any point to the most helpful spot without affecting the soundness of the defense

C=Defensive backs lined up to the left and right of the offense. these can be designated as field and boundary corners (i.e. one to the larger side of the field, and one to the sideline-shortened side of the field), or they can be designated to cover a specific target (i.e. #1 or #2 corner).


  • It should be noted that these are the designations for a defense to read the offense, not for an offensive system, where #1 receiver and #2 receiver mean something totally different.


  • #1: the first receiver counting from the outside in

  • #2: the second receiver, counting from the outside in

  • #3: the third receiver, counting from the outside in.

  • These designations will help when we later discuss "help" roles and rotating coverages. For now, it is enough to notice that regardless of the formation, 5 eligible receivers will always create the #3 receiver as a sort of "center" to the passing strength, an important concept for when we later look at the role of the Free safety.