In today's installment of the "NFL 101" series at Bleacher Report, former NFL defensive back Matt Bowen breaks down the basics of Cover 4 to give you a better understanding of scheme and execution at the pro level.

Click here for a breakdown of Cover 1.

Click here for a breakdown of Cover 2.

Click here for a breakdown of Cover 3.

Cover 4 (or "Quarters" coverage) is a four-deep, three-under zone defense that uses man-to-man principles while creating opportunities for both the free and strong safety to double (or "bracket") the No. 1 wide receivers.

Think of both cornerbacks and the two safeties in a standard four-deep look (cornerbacks align at seven to eight yards off the ball, safeties at 10-12 yards) with two underneath flat defenders and a linebacker playing the "middle hook" to wall off any inside breaking concept.

A coverage that was a core scheme for multiple defenses during my time in the NFL, Cover 4 has been exposed to an extent versus today's NFL offenses.

With play action to set the bait for safeties (leaving the cornerbacks exposed), plus spread looks that target the three underneath defenders in the short-to-intermediate route tree, I see much more Cover 6 (quarter-quarter-half) on tape compared to a straight "Quarters" alignment.

Today, using the All-22 tape, we will look at the base alignments, rules and responsibilities in Cover 4 while also breaking down some key calls (Cut, Box, Push, Zorro) that defenses will utilize based on the offensive formation and pre-snap alignment (bunch, reduced splits, etc.).

Base Cover 4

Let’s start by looking at the base Cover 4 alignment from the Buccaneers-Falcons matchup to discuss some key terminology and techniques in "Quarters" coverage.

As you can see here, the Bucs have their Posse/11 personnel (3WR-1TE-1RB) on the field in a 2x2 Doubles formation. The Falcons counter with their nickel sub-package (five defensive backs) playing Cover 4 in the secondary.

Here are the base rules you need to know when breaking down Cover 4:

- The two flat defenders (Sam 'backer, nickel in this example) "buzz" to any threat in the flat. They cannot be outleveraged and will "zone up" if there is no threat from No. 2/No. 3 underneath.

- The "Mike" linebacker ("middle hook" defender) is responsible for "walling off" any underneath crossing concept. Plus, he will carry/cushion No. 3 in a 3x1 formation to protect the safety playing quarters technique.

- Both cornerbacks align off (seven to eight yards) with an outside shade (outside eye) and pattern match the vertical release from No. 1 in the outside 1/4 (man-coverage technique). If there is no vertical stem from No. 1 (smash, shallow drive route), the CB will sink and look to help inside on a possible 7 (corner) route.

- Cornerbacks can play Cover 4 from a press alignment and jam/re-route on the release with help from the safeties inside in a quarters technique. As you can see below, both corners are in a press look with the safeties reading through the release of the No. 2 receivers (slot receiver, tight end).

- The strong and free safety play with a flat-foot read (no backpedal) through their run/pass keys (fill downhill versus the run) and align on the inside eye of No. 2 (inside slot receivers in this example).

They are responsible for playing the vertical release of No. 2 (pattern match) past a depth of 12 yards. However, if there is no inside vertical threat, both safeties will look outside to "bracket" No. 1 on a possible curl, dig or post (wheel to the hip of the receiver on the post).

Cover 4 vs. 3x1 Formation

The ability to defend the inside vertical from the No. 3 receiver out of a 3x1 formation is a major concern with playing Cover 4 in the NFL (think of the "999" route).

This puts stress on the strong safety (splits No. 2 and No. 3 versus a 3x1 formation) and the Mike linebacker underneath that has to carry/cushion the vertical seam from No. 3.

However, there is a technique defenses can use to limit that inside vertical by locking the backside cornerback ("solo" coverage) to allow the free safety to work/overlap No. 3 to the closed side.

Here's an example from the Titans-Texans tape with Posse/11 personnel on the field for Tennessee in a Doubles Slot formation.

Check out the open side of the formation with the cornerback in an off-man position. This is a "lock" call. He is now responsible to play that receiver in a man-to-man situation with the possibility of no help to the inside.

That allows the free safety to overlap any throw up the seam to the No. 3 receiver and take away the vertical concept while the strong safety widens to play over the top of No. 2.

Cover 4 "Cut" vs. 3x1 Formation (Reduced Split)

To the backside of a 3x1 formation, Cover 4 teams can "cut" the X receiver (based on game plan) if he aligns in a reduced (or "nasty") split tight to the core of the formation.

This allows the free safety to drive downhill on a possible shallow drive route or "over" route (intermediate crossing route) with the cornerback replacing him in the deep 1/4.

Here's an example from the Chiefs-Colts matchup in the Wild Card Game this past season with Posse/11 personnel on the field for Indianapolis in a Doubles Slot formation (3x1).

With the X in a reduced split, the free safety will drive downhill versus a hard, inside release and play to the upfield shoulder of the receiver (can't overrun the route on a possible whip/stop).

This leaves the backside cornerback in a position to sink and look for work to the closed side of the formation for a possible vertical from No. 3 (tight end).

As I said above, the defense can put the cornerback in a "lock" or "solo" call versus the backside X in a 3x1 formation (allows the free safety to work to No. 3 vertical).

However, the "cut" call is an aggressive way to eliminate the shallow drive route from the backside X receiver while putting the cornerback in a position to read the quarterback and help to the passing strength of the formation.

Cover 4 "Box" vs. Bunch Alignment

In order to counter the multiple bunch formations (three receivers close together) from NFL offenses, Cover 4 teams can use the "box" call.

It's a technique that creates a four-on-three matchup for the defense with two underneath defenders (first "in" and first "out") and two deep defenders (second "in" and second "out") to form the shape of a box. 

This allows the defense to play combination routes out of the bunch (Spot, Hi-Lo, etc.) while creating opportunities to sit hard at the break point with protection over the top.

Let's look at an example of the "box" technique from the Rams-Texans matchup with Posse/11 personnel on the field in a wide bunch alignment for St. Louis.

The Rams motion inside to create the bunch look, and the Texans check to a "box" call (hand signal). This allows Houston to use the two underneath defenders and the safety/corner combo over the top to play the Hi-Lo/7 combination by "matching" to the route concepts.

This is how it plays out on the field with the "inside" underneath defender matching to the shallow drive, the "outside" underneath defender in a position to drive the running back on the checkdown, the safety playing top-down versus the dig route and the cornerback sinking over the top of the 7 (corner).

Cover 4 "Push" vs. Slot Formation

The "push" call (alerted before the snap) shows up versus offensive slot alignments with the strong safety working to the open side of the formation to "rob" the inside smash or curl while allowing the free safety to widen outside.

A great technique to use inside of the red zone (field shrinks for the offense), the strong safety must first read the release of the tight end for his run/pass keys and then "push."

This is an example of the "push" technique from the Texans-49ers matchup with Ace/12 personnel (2WR-2TE-1RB) on the field for San Francisco in a Slot Open formation.

If the tight end blocks, or releases underneath, the strong safety will work to the closed side of the formation.

This is important versus a slot look, or in this case a Slot Open formation, as it allows the safety to "push" to a possible vertical from No. 3 or play as a "robber" to jump the smash/curl (as simulated in the diagram).

The key is the quarterback. Often, he will not account for the backside safety to push underneath to that inside curl zone. And if the safety plays with a flat-foot read through the tight end release, while driving downhill on the throw, he can steal one in this situation.

Cover 4 "Zorro" vs. Tight Split/Stack

If you show Cover 4 on tape, then expect to see the "scissors" route (corner/post combo) from a tight split or stack look.

The idea is to force the strong safety to chase the 7 (corner) route (from a trail position) while removing any inside help for the cornerback playing with outside leverage versus the post.

However, the "zorro" call (alerted before the snap) allows the secondary to "pass off" the "scissors" route while eliminating a possible explosive play for the offense.

Let's take a look at how it would play out using the tape from the Texans-49ers matchup with Ace/12 personnel on the field in a 2x2 Doubles formation.

With the tight split, the Texans can alert the "zorro" call before the snap to protect versus a possible "scissors" route with the safety taking the wide receiver on the post and the cornerback sinking outside to play the 7 route from the tight end.

A call that is crucial inside of the red zone (high alert for scissors combination), Cover 4 teams have to use this "zorro" technique anytime they see a reduced split to the tight end, a stack or a "vice" alignment (double stack).

The No. 1 Cover 4 Beater in the NFL

If you want to beat Cover 4, then run the "Pin" route (Post-In combo).

The idea is to remove (or occupy) the strong safety underneath with the dig route (eliminate the inside help) while targeting the outside leverage of the cornerback on the deep post.

Also called a "Mills" concept (Steve Spurrier's playbook), the "Pin" route can include a "dino" stem from the No. 1 receiver (stem to the corner, break back to the post) to widen the cornerback and create even more separation back to the inside.

Here’s a look at how the "Pin" route plays out versus Cover 4 using the tape from the Chiefs-Colts Wild Card Game with Posse/11 personnel on the field for Indianapolis in a 2x2 Doubles alignment.

With T.Y. Hilton in the slot, the Colts will set the bait for the strong safety on the dig route and stem the No. 1 receiver to the post from a plus-split alignment.

This forces the strong safety to attack Hilton downhill on the dig while leaving the cornerback exposed from an outside leverage position (with no help to the inside).

Look at the opportunity for quarterback Andrew Luck and the Colts down the field.

With the safety now removed versus Hilton, Luck can target the deep post because of the receiver's inside leverage position. 

The result is an explosive play for the Colts on a classic Cover 4 beater. 

Up next in the "NFL 101" series: An Introduction to Zone Blitzes.

Seven-year NFL veteran Matt Bowen is an NFL National Lead Writer for Bleacher Report. 

Follow @MattBowen41