For TCU Horned Frogs head coach Gary Patterson, the development of his 4-2-5 defense has become his calling card and represents a significant contribution to defensive strategy as more and more teams have to consistently defend spread offenses and play nickel defenses.

Some of the genesis of the defense came while coaching at UC-Davis in 1986, where he picked up blitz packages and much of the current playbook, including a lot of the terminology, but it wasn't until a stop at Utah State from 1992-94 with defensive coordinator Dick Bumpas that Patterson began to truly embrace the 4-2-5, which he had helped Dennis Franchione run at Pittsburg State in 1988. In 1996, Patterson got a chance to helm the defense himself after Franchione hired him at New Mexico to help him install the 4-2-5, which Frachione had been running for some time.

After Patterson followed Franchione to TCU in 1998, he took over the program in 2000 when Franchione bolted for Alabama.

There are a number of defining characteristics of the Patterson 4-2-5 defense, which he bases on the idea of being multiple, but also simple enough that it is easy for players to understand despite all the moving parts. The secondary is separated from the assignments of the front six and then cut in half, with the ability to play man coverage on one side and a zone on the other side in a number of different combinations.

At New Mexico and during many of the years at TCU, Patterson used the defense because he felt it allowed less talented defenders to compete with more talented offenses operating in space because of its flexibility -- for instance, the natural alignment of the secondary makes it extremely easy to disguise a number of blitz packages without the offense being able to get a good pre-snap read on where defenders are going to be once the play starts.

For Patterson, his defense boils down to creating confusion at the line of scrimmage, playing with great leverage, establishing an eight-man front, and establishing a pressure package. Pretty basic stuff for any defense, but Patterson goes about it differently than most because of those separate calls for the front and the secondary.

The leverage is an important part of what makes the defense work. Though the Horned Frogs don't necessarily pack the box in the way that other teams do, the alignment of the weak safety and the strong safety just outside the box allows the safeties the proximity to the line of scrimmage to make plays against the run, but not give up anything in coverage while doing it.

In fact, the goal of the front six is to direct everything outside to the safeties, who are taught independently of the cornerbacks with a heavy focus on leverage and angles. Patterson stresses playing "inside and in front" -- the safeties are taught that once a ballcarrier declares their intention, they are not to allow them to cross their face. As a result, TCU defenses are known for their ability to flow hard and quickly to the ball and arrive there with numbers.

The eight-man front helps fulfill one of Patterson's central tenets, which is stopping the run first and foremost by outnumbering the offense at the point of attack. And since the front is disconnected from the secondary, but focused on pushing the ball to the safeties, the TCU front six will often slant or stunt to create confusion in opposing blocking schemes. In fact, there are 15 stunts and 15 twists in the Horned Frog playbook -- the front wreaks havoc and the safeties are there to clean everything up.

And it works, as TCU led the nation in total defense in 2008, 2009, and 2010.

Like most schools, TCU also has an extensive blitz package. Again, that package is aided by the disguise allowed from the alignment of the weak and strong safeties next to the line of scrimmage. Patterson is not exactly risk-averse, as this isn't a bend-but-don't-break defense, but he also believes that the threat of blitz is more valuable than a blitz -- it has probably gotten a team out of their ideal play and the defense hasn't committed resources directly towards the quarterback.

When blitzing, the Horned Frogs use zero or man coverage, a departure from the hybrid coverages used in the base defense. This is the best time for an offense to run some of the man-beater routes that TCU can typically take away with their base defensive concepts in the secondary. The problem, of course, is how little time the offense will have to get the ball out before the blitz likely gets to the quarterback.

Against the pass, the base TCU defense is the Cover 2 "Read", meaning that the free safety will declare which side of the formation is the read side. However, the Horned Frogs will also use Blue and Cover 5 looks against one-back formations.

To describe the Cover 2 Read defense simply, the cornerbacks are responsible for covering the deep threats, which allows them to take away post and corner routes, while the weak and strong safeties play curl/flat to take away the underneath passes. On the read side, this means that if the outermost receiver runs a shallow route on a smash concept that is difficult to deal with for most Cover 2 defenses because the safety is responsible for taking away the corner route, the safety will jump the underneath route from the No. 1 receiver while the cornerback drops to undercut the inside receiver (No. 2 receiver) breaking outside and the free safety has deep responsibility.

On the other side, the cornerback is responsible for getting over the post of the outermost receiver (the no. 1 receiver), while the weak safety also plays curl/flat. The linebacker to that side will carry the innermost receiver (the no. 2 receiver) if they get vertical.

Note that the descriptions in the preceding two paragraphs are against a 2x2 alignment for the offense. More information on this coverage and diagrams are available here.

Another significant part of the TCU coverage scheme is Cover Blue, which plays the cornerbacks in bail techniques with their eyes on the No. 2 receiver and the quarterback, allowing them to jump underneath routes. Meanwhile, the safeties also take their cues from the No. 2 receiver, but are responsible for covering a lot of ground on these routes, as they have to be able to play man-to-man against the No. 2 receiver getting vertical, but also provide help to the cornerback on deep routes if the No. 2 stays to block (as a tight end) or runs a short, outside-breaking route.

The final coverage that TCU uses is Cover 5. The weak and free safeties are responsible for covering half the field deep, while the strong safety and both linebackers drop at depth to deal with vertical routes and the two cornerbacks play cloud technique -- zone coverage on anything underneath or in the flats. However, Patterson will not use this defense if he's afraid of vertical threats from opposing receivers, but rather to provide coverage in the flats if there are running backs leaking out causing problems.

Multiple, but simple enough to be able to start a player like Chris Hackett at safety in 2012 as a freshman, TCU can from the same defensive call outnumber the run game and still have the ability to cover the deep throws like corners and posts that teams often use to produce big plays in the passing game. The pattern-matching in the secondary also keeps opposing offenses from being able to exploit specific man-to-man match ups without the liability of the holes often found in most zone defenses.

The defense is one that Patterson has molded and refined over the years and despite often failing to recruit at a high level, the TCU head coach has a deep understanding for the type of athletes that he wants to target and he does an excellent job of developing his players, resulting in a defense that is incredibly hard to pick apart.