Zone defense in American football

Zone coverage, (also referred to as a zone defense) is a defense scheme in American and Canadian football used to protect against the pass.

Zone coverage schemes require the linebackers and defensive backs to work together to cover certain areas of the field, making it difficult for the opposing quarterback to complete passes. Zone defenses will generally require linebackers to cover the short and midrange area in the middle of the field, in front of the safeties. In the case where one or two linebackers blitz, the remaining linebacker(s) expands his zone to cover the zone responsibilities of the vacating linebacker(s). Often, blitzing will leave larger holes in the pass defense, but it is a gamble the defensive coordinator wants to make to pressure the quarterback into a poor decision and hopefully an interception or at least an incompletion.

Coverage Shells
In the following, "cover" refers to the "shell" that the defense rolls into after the snap of the ball, more specifically the number of defenders guarding the deep portion of the field.

Cover 1
Cover 1 schemes employ only one deep defender, usually a safety. Many underneath coverages paired with Cover 1 shells are strictly man-to-man with LBs and defensive backs each assigned a different offensive player to cover. By using only one deep defender in Cover 1, the other deep defender is free to blitz the quarterback or provide man-to-man pass coverage help.

Cover 1 schemes are usually very aggressive, preferring to proactively disrupt the offense by giving the quarterback little time to make a decision while collapsing the pocket quickly. This is the main advantage of Cover 1 schemes--the ability to blitz from various pre-snap formations while engaging in complex man-to-man coverage schemes post-snap. For example, a safety may blitz while a CB is locked in man coverage with a WR. Or the CB may blitz with the safety rotating into man coverage on the WR post-snap.

The main weakness of Cover 1 schemes is the lone deep defender that must cover a large amount of field and provide help on any deep threats. Offenses can attack Cover 1 schemes with a vertical stretch by sending two receivers on deep routes, provided that the quarterback has enough time for his receivers to get open. The deep defender must decide which receiver to help out on, leaving the other in man coverage which may be a mismatch.

A secondary weakness is inherent in its design: the use of man coverage opens up yards after catch lanes. Man coverage is attacked by offenses in various ways that try to isolate their best athletes on defenders by passing them the ball quickly before the defender can react or designing plays that clear defenders from certain areas thus opening yards after catch lanes.

Cover 1 can also be used to confuse a Quarterback. For example setting a safety with a zone below the deepest safety can make Cover 1 look like Cover 2. Cover 1 also allows a defense to be in good position to stop the run. cover 1 schemes can also be ran with cornerbacks or linebackers.

Cover 2
In traditional Cover 2 schemes the free safety (FS) and strong safety (SS) have deep responsibilities, each guarding half of the field. The NFL's Tampa Bay Buccaneers, Indianapolis Colts, and Chicago Bears run a variant of this defense called the Tampa 2.

Cover 2 can be run from any seven-man defensive fronts such as the 3-4 and the 4-3. (It is difficult to implement Cover 2 from an eight-in-the-box front, because the strong safety or someone replacing him is usually the eighth man). Various "underneath" coverage played by cornerbacks and linebackers may also be implemented. For example, Cover 2 Man means 2 safeties have deep responsibility while the cornerbacks and linebackers follow their offensive assignment in one-on-one coverage. The NFL's San Diego Chargers inherited a base Cover 2 Man 3-4 from Wade Phillips. Cover 2 can also be paired with underneath zone schemes: Cover 2 Zone refers to 2 safeties with deep responsibility but now the CBs and LBs drop back into specific coverage zones where they defend passes only in their assigned area.

Teams that play Cover 2 shells usually ascribe to the "bend-but-don't-break" philosophy, preferring to keep offensive players in front of them for short gains while limiting long passes. This is in stark contrast to a more aggressive Cover 1 type scheme which leaves the offensive team's wide receivers in single man-to-man coverage with only one deep helper. By splitting the deep field between two defenders, the defense can drastically reduce the number of long gains.

The main weakness of the Cover 2 shell occurs in the middle of the field between the safeties. The safeties attempt to gain width upon the snap of the ball to cover any long passes to quick wide receivers down the sideline. This movement creates a natural hole between the safeties that can be attacked. By sending a receiver (usually a tight end) into the hole, the offense forces the safety to make a decision: play the vulnerable hole in the middle of the field or help out on the wide receiver. The quarterback reads the safety's decision and decides on the best matchup (i.e. which mismatch is better--TE vs S or WR vs CB). Also many NFL teams find a weakness between the corner and the safety on a deep out route. There is a little gap in between and the best quarterbacks can hit the gap with the right timing.

Cover 2 also clogs up the QB's throwing lanes. A linebacker can be assigned deep to cover the area between the safeties and the linebackers, similar to Cover 3. By dropping a CB to the one of the zones can allow a safety to blitz or drop down into the box area.

Cover 3
Cover 3 refers to 3 deep defenders each guarding one-third of the deep zone. Cover 3 schemes are usually used to defend against passes, mainly those towards the deep middle of the field. Unlike Cover 2 schemes that create a natural hole between safeties, Cover 3's extra deep defender is able to patrol the middle area effectively.

The most basic Cover 3 scheme involves 2 CBs and a safety. Upon snap, the CBs work for depth, backpedaling into their assigned zone. One safety moves toward the center of the field. The other safety is free to rotate into the flat area (about 2-4 yards beyond the line of scrimmage), provide pass coverage help, or blitz.

One of the biggest benefits of the cover 3 coverage scheme is the ability to walk the SS up into the box with minimal to no changes in the coverage due to the pre-snap center field position of the FS. This enables the defense to play both man and zone coverage out of an 8 man front while cover 2 schemes allow only for man coverage with 8 man fronts. The New England Patriots were notorious for using SS Rodney Harrison as more of a third OLB than a coverage safety and regularly employ cover 3 coverages.

Cover 3 schemes are susceptible to short, timed passes to the outside due to the hard drop of both CBs. This puts pressure on the OLBs to get into their drop quickly. Another disadvantage of cover 3 schemes is they are relatively easy to diagnose by opposing QBs. Because of this teams will often employ slight wrinkles in their coverage to confuse offenses. An example of this includes employing man coverage on one side and zone on another or swaping coverage zones between defenders.

Teams can also "roll" their safeties to one of the sides of the field, by send only and one CB into coverage and rolling a safety to outside.

Cover 4
Cover 4 refers to 4 deep defenders each guarding one-fourth of the deep zone. Cover 4 schemes are usually used to defend against deep passes. (See Prevent defense).

The most basic Cover 4 scheme involves 2 CBs and 2 safeties. Upon snap, the CBs work for depth, backpedaling into their assigned zone. Both safeties backpedal towards their assigned zone.

As with other coverage shells, Cover 4 is paired with underneath man or zone coverage in its most basic form.

The main weakness of Cover 4 shells is the retreating defensive backs. Since the DBs are working for depth, short pass routes underneath can isolate them on a wide receiver near the sideline with little help.

Cover 6
Cover 6 refers to one side playing "Cover 2 rules", and the other playing Cover 4 rules". The coverage is named Cover 6 because 4+2=6. Cover 6 is used to provide a mismatch in coverage for the offense.

Schemes:

On the strong side, the corner and safety play "Cover 4 rules", which as above the corner and safety each have a quarter of the field working for depth in their zones. The "Sam" linebacker will be dropping outside to cover the flats. If in 3-4 the Middle Backer will cover that sides hook to curl if not blitzing.

On the weak side, the corner and safety play "Cover 2 rules", which as above the corner stays home in the flats, and the safety covers the deep half. The Will backer will play hook to curl or blitz depending on the call. If in 3-4 usually the will or the Middle Backer will blitz from that side.

The Cover 6 is also good for calling a corner blitz from the weak side, and having the backer cover flats instead.

The main weakness of cover 6 occurs similarly to cover 2 where you send receivers into the hole between the safeties (but a smaller hole). Also any deep backside pass with a free safety taking deep half, and not having the same help.

Teams that run cover 6 generally do not start in 6, they usually check into 6 if one side has twins and the other has WR & TE, or it could be based on any judgement call of the defense.

Cover 0
Cover 0 refers to pure man coverage with no deep defender. Similar to Cover 1, Cover 0 has the same strengths and weaknesses however employs an extra rusher at the expense of deep coverage help leaving each pass defender "on an island" with his man.

Tampa 2
Tampa 2 refers to a style of defense played by the Tampa Bay Buccaneers and implemented by its coaches, Tony Dungy, Lovie Smith, and Monte Kiffin, in recent years. Because of its success it has become popular with many professional and college teams. It blends the Cover 2 and Cover 3 defenses by having two defensives backs, usually the safeties, in deep coverage on either side of the field, and a middle linebacker covering the medium to deep middle. Its benefit over the Cover 2 is that the sidelines and middle of the field are better protected against deep threats, with the drawback being a larger open area in the short middle of the field underneath the middle linebacker. Its benefit over the Cover 3 is that it only dedicates two defensive backs to deep coverage rather than three, allowing for better protection against short outside routes. The Tampa 2 generally requires a quick middle linebacker who is capable of staying with tight ends and wide receivers in pass coverage.