Football 101: Coverage Shells

The purpose of this article is to serve as an easy to understand guide to defensive coverage schemes for those who are learning some of the basics about football. Phinsider overlord managing editor Kevin Nogle wrote a great introduction to defensive line techniques in his post, "Football 101: Defensive line gap techniques." I recently posted my own breakdown of the linebacker roles in the Dolphins' 4-3 defense in my creatively named FanPost, "Brief Overview of the 4-3 Linebacker Roles in our Base Defense."

Those are primers on what defensive linemen and linebackers are generally asked to do. The next group to discuss is the defensive backs, who are the safeties and the cornerbacks. To keep things simple, these coverage shells will be described as if they are being run by the Dolphins in their favored pass-stopping "nickel" package, which features 4 defensive linemen, 2 linebackers, and 5 defensive backs (hence the term, "Nickel"). I will avoid discussing the technical nuances of each coverage shell because the goal is to provide a decent summary of the 6 most popular coverage shells in just one article.

Cover 0

This is the ultimate, "I simply don't respect your ability to throw the ball" coverage. No defensive backs are asked to drop back into zones, so both safeties are free to be aggressive near the line of scrimmage.

The advantage is that it allows every single member of the 7-man coverage unit (safeties, linebackers, and cornerbacks) to be used in man coverage. On any given play, the offense can send up to 5 players to run receiving routes because out of the 11 players, six of them (the quarterback and 5 offensive linemen) stay behind the line of scrimmage on passing plays. Cover 0 allows a defense to assign 5 defenders to cover those 5 players man-to-man. Meanwhile, the 7-man coverage unit still has 2 defenders "left over" to either blitz/track the QB or roam near the line of scrimmage. The 2 leftover defenders could be safeties or linebackers or cornerbacks or a mix of 2 of those 3 position groups.

Defenses almost always blitz with this coverage shell because giving the quarterback too much time in the pocket guarantees at least one receiver will get open against man coverage without safety help. This type of coverage shell is excellent at shutting down shorter routes as well as handoffs because of all the "traffic" it creates with all 11 defenders near the line of scrimmage, but it leaves the defense dangerously vulnerable to deep passes, which is why teams usually only use cover 0 in special situations such in the redzone.

The most recent famous example of a team playing cover-0 far away from the endzone comes from the Tim Tebow-led Denver Broncos playoff game in 2012 against the Pittsburgh Steelers. Tebow is a quarterback with many strengths including vocal leadership and running the ball, but "consistent accuracy on intermediate throws" is not one of them, so the Steelers chose to be very aggressive in stopping the Tebow-led rushing attack by using cover-0 sometimes. In particular, they used it at the start of overtime.

(Click on each picture to view a high resolution image if you're having trouble seeing details)

Before this play started, the Steelers had only 1 safety playing deep (red box in the image above). The strong safety (orange box) is lined up next to their inside linebackers. If you look closely, you'll see the free safety (red box) has begun running up towards the line of scrimmage before the snap.

As the image below shows, by the time Tebow gets the ball in his hands, multiple Steelers defenders are matched up man-to-man, and neither safety (orange or red box) is playing deep.

Both safeties are within 4 yards of the line of scrimmage at the time of the snap. If the Broncos run the ball against this front, they're very unlikely to gain yards due to the safeties being nearby to provide run support. The defensive back that is furthest downfield is a cornerback who is matched up man-to-man against the boundary wide receiver (each outlined by a black box), and besides that wide receiver, no Broncos receiver is open soon after the snap because of how aggressively the defensive backs have positioned themselves.

For those who don't know what happened next, Tebow made a nice pass to Demaryius Thomas, who broke one tackle and scored an 80 yard touchdown because the Steelers had no defensive backs playing deep to stop him. You can watch the famous play here.

You'll sometimes see Dolphins defensive coordinator Kevin Coyle call for this shell if the Dolphins defense is backed up near the end zone, with the logic being that there's not much benefit in playing any deep safeties when the offense is so close to the endzone that they will instantly score a touchdown if they can complete a 5 or 10 yard pass. Otherwise, you're almost always going to see at least 1 defensive back (the free safety) dropping back deep. So what type of coverage shell is it when only the free safety drops back deep?

Cover 1

This is the, "I want to be aggressive in stopping the run and short throws in any direction, while giving my team a last line of defense" coverage. In this shell, the free safety is asked to drop into a deep zone, and his mission is to make sure he stops any players from getting past him. The strong safety, cornerbacks, and linebackers are generally in man-coverage.

Even with the free safety dropping deep, that leaves 6 "coverage" defenders available to play man-to-man, and as I wrote earlier, only 5 offensive players can run routes downfield. That leaves 1 "free defender" to do one of two things - either blitz or drop into zone coverage. Coyle is a defensive coordinator who loves to blitz, so it's not uncommon to see a linebacker or defensive back sent on a blitz as the free safety drops into a deep zone while the remaining 5 linebackers/defensive backs drop into man coverage. That's cover-1.

However, if the Dolphins' 4-man front is getting decent pressure, Coyle sometimes won't call a blitz. When that happens, this is probably strong safety Reshad Jones' favorite coverage shell. When Jones is not asked to cover a tight end or running back in man-coverage, he can "roam" as a strong safety and try to make big plays using his instincts either in run support or in intercepting passes downfield, as he did against Joe Flacco and the Ravens when he scored a pick-6 last year. Jones was free to intercept the batted pass because he had dropped into an intermediate zone, while Clemons was asked to drop into a deep zone, and the other members of the Dolphins' coverage unit were in man coverage. This type of cover-1 is called "cover-1 robber" because it allows 1 man to act as a potential "thief" on short-to-intermediate passes.

In the screenshot below, you see Jones (#20) standing in an intermediate zone as Dolphins defenders such as Jelani Jenkins (#53) and Phillip Wheeler (Dolphins player closest to Jones) are in man coverage. Misi, Vernon, Odrick, and Jordan are the Dolphins' 4-man rush (Misi was lined up as a DE as part of the Dolphins' "speed package" that features 3 DEs and just 1 DT).  Jordan is about to beat Bryant McKinnie (#78) with a speed rush to force a bad throw by the quarterback.

Yes, Wheeler is clearly committing an illegal contact penalty by grabbing Ravens' #17 far downfield, but the referees thankfully didn't see it. In the next screen shot, you see Jones waiting for the tipped ball to land at the 25 yard line, as Clemons moves forward from his deep zone, with Nolan Carroll in man-coverage near the sideline.

For animated .gif images like the one below, simply move your mouse over the image to begin the animation.

My super-clever way of showing when the ball has been snapped is that the circle representing the Center becomes empty while the circle representing the Quarterback becomes brown, like a football. Genius, I know.

That purple area in the middle of the field is the "Reshad Jones Party Zone." When Jones is forced to play man-defense, his attempts to gamble have disastrous consequences because the defensive playcall counts on Jones to cover his man. When Jones roams in cover-1 and isn't assigned to any man (meaning he's in his "party zone"), he's able to gamble knowing that he has a free safety playing deep center field to prevent huge plays and that each potential receiver is already being covered by a teammate of his. Jones had a lot of highlight reel plays in 2012 against both the run and the pass since Coyle felt comfortable letting Jones roam. Unfortunately, because our linebackers in 2013 weren't very good in coverage, Coyle needed Jones to play more man-to-man defense. Jones was frequently victimized in man-coverage in the first half of 2013 until he cut down on his gambling tendencies, at which point, he was more reliable in coverage but no longer generated big plays.

Overall, Cover 1 is a good shell if you want to be aggressive without leaving your defense completely exposed without deep safety help. Every receiver gets assigned a defender, and it leaves a defensive player free to blitz or roam. However, against a team with at least 2 threats to go deep, leaving only 1 safety deep means only one of those threats can be contained with safety help. Because of that, some teams choose to play 2 safeties deep.

Cover 2

This is the, "I want to free up my cornerbacks to be aggressive" coverage. By dropping both safeties deep downfield, cornerbacks and linebackers playing "underneath" can play zone, press-man, or off-man coverage, and they can be asked to be aggressive in trying to create interceptions since they have the "safety blanket" of not just one, but 2 safeties playing deep downfield. Wonderful, right? Why doesn't everybody play this?

There are two main weaknesses to Cover 2. First, the best defensive back against the run (the strong safety) is asked to play far from the line of scrimmage, limiting his ability to help on running plays. Second, if you look closely, you'll see there's some space in between both safeties deep downfield. When pointing out the danger in cover-1, I mentioned the possibility of a team having 2 "deep threat" wide receivers that couldn't both be contained downfield by 1 safety. Well, what if a team has 2 "deep threat" wide receivers plus a very athletic pass-catching tight end?

Tight ends who can "stretch the seam" (run well near the middle of the field) are "cover-2 killers." If each safety commits to an outside wide receiver, the "seam threat" tight end is left one-on-one against a linebacker, and most NFL linebackers struggle to defend athletic tight ends. Defending the tight end with a nickel cornerback forces a linebacker to cover the slot receiver if the offense is using 3 wide receivers and 1 tight end, which is very common. Using a dime cornerback on the tight end so the nickel cornerback can guard the slot receiver leaves your defense incredibly vulnerable to the run given that only 1 linebacker is left on the field. With every NFL team trying to use tight ends to stretch the seam like Tony Gonzales or Antonio Gates, the cover-2 shell has become less popular.

Cover-2 teams have tried to improve coverage down the seam by having a third player drop back into a deep zone. When it's a linebacker, it's called a "Tampa 2 defense." An ideal prototype linebacker that even fans new to football should know is Brian Urlacher, who was converted from safety to linebacker in college and was a freak athlete for his size. He was big and strong enough to fulfill the run stopping duties of a middle linebacker while also having enough speed and coverage instincts to effectively drop into deep zone coverage. Urlacher's talent allowed the Tampa 2 scheme of former Bears' head coach Lovie Smith to remain highly effective even as other teams played less cover-2.

Still, not every team has a Brian Urlacher-type linebacker who can both stop the run and drop back deep downfield, so how do you defend teams with multiple deep threats at wide receiver and tight end while not fatally compromising your run defense by using dime (4 defensive linemen, 1 linebacker, and 6 defensive backs)?

Cover 3

This is the, "I want to take away deep throws and runs up the middle" coverage. It may seem weird for a coverage shell to accomplish both, but once you see it in action, it makes sense.

The two boundary cornerbacks usually line up a few yards downfield before the snap, then at the snap, they drop back into deep zone coverage near each sideline. The free safety drops back into deep zone coverage in the middle of the field. The strong safety moves forward, putting him in position to help on any handoffs. The advantage of cover 3 is that it asks the 3 best players in coverage (the boundary cornerbacks and free safety) to drop into deep coverage, while it has the defensive back who is best in run support (the strong safety) move towards the line of scrimmage to help stop the run. Those strengths are why this is a very popular coverage shell. The strong safety, linebackers, and nickel cornerback can play either man or zone, but more frequently are asked to play zone, as shown above.

However, cover 3 does have 1 key weakness. While the entire deep area of the field is pretty well covered by 3 defensive backs, and the shallow-middle of the field is defended well with the strong safety dropping into support, both "flanks" of the defense (shallow areas near each sideline) are left vulnerable early in each play. Those outside areas near the line of scrimmage are defended either by a linebacker or defensive back who normally begins the play positioned near the middle of the field and has to move to the outside as the boundary cornerbacks run downfield. Quick, short throws to the outside that take advantage of the boundary cornerbacks dropping into deep zones are used to defeat cover 3. Those are high percentage throws, and those drop-backs by the cornerbacks open up the potential for good yards after the catch.

"But wait, don't the Seattle Seahawks play a lot of cover 3? Why don't they give up easy throws to the outside?"

The Seahawks use a variety of coverage shells, but their most common shell also happens to be their most uniquely effective, which is "press-cover 3." That system asks the boundary cornerbacks to first jam the wide receivers at the line of scrimmage before dropping back far downfield into their assigned zones. By jamming the wide receivers, the cornerbacks can prevent the quarterback from attempting a short throw to the outside as well as disrupt the timing on any downfield throws. The potential danger of this tactic is that the cornerbacks, after attempting to jam the wide receivers, could fail to drop back far enough downfield to prevent deep passes from being completed. They don't have the benefit of a "head start" downfield because they began the play very close to the line of scrimmage to try to jam the receiver.

That's where All-Pro free safety Earl Thomas comes in. It's Thomas' job to "bail out" any Seahawk cornerback who fails to jam his wide receiver effectively and allows the receiver to outrun him downfield. Although technically a cover-3 defense, the Seahawks' press cover-3 is a lot like the cover-1 scheme that was discussed earlier because it calls for the cornerbacks to be more aggressive early in the play. Thomas' ability to defend the deep middle third of the field by himself while having the elite range to also bail out his cornerbacks on either sideline if they've been beaten downfield is why some consider Thomas to be the best free safety in the NFL. Just as Brian Urlacher's skillset made the Bears' Tampa-2 work, Thomas' unique talent allows the Seahawks to prevent big plays like a cover-3 defense while also stopping shorter plays like a cover-1 defense.

Cover 4

This is "Prevent Defense." BORING. 4 defensive backs (the two safeties and 2 boundary cornerbacks) drop into deep zones, and their objective is to make sure no receivers get past them. With 1/3 of the defense playing deep downfield, this coverage shell is very good at stopping deep passes but very bad at preventing short to intermediate throws or any running play to the outside.

Assuming the defense has 4 men rushing the passer, asking 4 players to drop into zone leaves just 3 defenders (3 linebackers or 2 linebackers + a slot corner) to defend the short-to-intermediate area, and most of those defenders begin the play near the middle of the field. That positioning issue is why this defense is more vulnerable to outside runs than inside runs. VERY risk-averse defensive coordinators will sometimes only rush 3 defensive linemen to allow the fourth defensive linemen to drop into zone coverage as well. However, Coyle usually rushes at least 4 players, even in late-game situations, which I personally like to see.

Cover 6

Cover 6 is a hybrid shell - Half of the field is defended similar to a cover 4 shell, and half is defended like in cover 2.

Cover 2 + Cover 4 =  Cover 6

The concept behind this shell is that very often, the ball isn't snapped in the middle of the field. It could be closer to the left hashmarks or the right hashmarks. Therefore, if the ball is located closer to the right hashmarks, then the defense is less exposed if they play only 1 deep safety (the free safety) on that side of the field, so this side closer to the boundary will be the "cover-2" side. On the opposite side of the field, the defense will play 1 deep safety (the strong safety) and 1 deep cornerback because there's more space that needs to be defended, and 2 defensive backs playing deep on 1/2 of the field is similar to a "cover-4" look.

In that .gif, you see cover-4 on the offense's left, and cover-2 on the offense's right.

In terms of benefits, the strong safety is more able to provide run support than in cover-2 because he's splitting his deep half of the field with a boundary cornerback. Rather than dropping back downfield AND sprinting towards the sideline, the strong safety only has to drop deep if it's a pass. Meanwhile, the free safety on the other side must drop back deep and get to the sideline because he's the only defensive back on that side of the field. This defense looks almost as good as stopping deep passes as cover 4 while providing more run support than cover 2. So why doesn't everybody use this constantly?

Well, the weaknesses depend on which side of the field you're looking at. First, let's focus on the side of the field that has more space in between the ball and the sideline (the cover-4 side). In a base defense (3 linebackers), a linebacker (instead of a nickel cornerback) is asked to cover the entire shallow area near the sideline because both the boundary cornerback and strong safety are dropping deep, leaving it vulnerable to a quick pass to that area before the linebacker can get there.  Replace the linebacker with the nickel cornerback, and that area is very vulnerable to an outside run because the boundary cornerback on that side drops back immediately at the start of the play, leaving only 1 nickel cornerback in between the running back and the sideline. Last but not least, with the strong safety given the green light to be aggressive in run support, play-action can create a one-on-one coverage situation deep down the sideline if the strong safety gets fooled. As for the "short" side of the field being defended similar to cover 2, the seam is vulnerable between the cornerback and linebacker, just like in the cover-2 scheme, so smart teams will attack that area with a tight end.

Conclusion

There's an incredible amount of additional detail available for each coverage scheme, such as the exact locations of each player on the field (ex. how many yards from the line of scrimmage) as well as the different ways defenses can try to confuse quarterbacks while using each coverage shell. However, this article is designed to be simply an introduction to each coverage shell, and why coaches choose to either use them or avoid them. It's a lot of information to process at first, but hopefully in the future, if you forget some details about each shell, you can refer back to this article and especially the images to refresh your memory.