Editor's note: As architect of Texas A&M's Wrecking Crew defense (1989-93), Notre Dame defensive coordinator (1994-96) and head coach of the Irish (1997-2001), Bob Davie has been recognized as a top X's and O's coach. This season, Coach Davie will analyze offensive and defensive schemes as part of his season-long course on football for ESPN.com. Each week, he will break out the chalkboard and break down the X's and O's in college football.

It is a necessity that every good offensive football team has a diversified screen package to combat today's attacking and blitzing defensive philosophies. Screens are an important element in successful offensive football for several seasons: (1) Screens take advantage of athletic wide receivers and running backs and make defenders tackle in the open field matching the wide receivers and running backs on linebackers and safeties; (2) Screens slow the pass rush and make defensive linemen eye for screens; (3) Screens attack zone blitz teams that give up an underneath zone in pass coverage; (4) Screens give the offense a chance for big plays against man-to-man blitz schemes.

The reason screens are so successful in college football is that offenses take advantage of a very important NCAA rule. In college football, offensive players are allowed to block downfield while the ball is in the air if the ball is caught behind the line of scrimmage. This is a major advantage to the offense because offensive players can actually pick defenders while the ball is in the air. Whether the defense is in zone or man-to-man coverage, this puts them at a tremendous disadvantage. (In the NFL, you are not allowed to block downfield while the ball is in the air regardless of where it is caught.)

Each of today's offenses feature several types of screens. We will explain the following screens: (1) the bubble screen, (2) jailbreak screen, (3) the traditional slow screen, and (4) the crack screen.

Bubble screen vs. 3-deep or soft man-to-man
The bubble screen is a wide receiver screen where the receiver actually bubbles away from the line of scrimmage and the quarterback. The most common form involves the other wideout picking the defensive back and giving the receiver a chance to run after the catch. The wide receiver bubbles back to allow this to time out. This is a great route against a 3-deep zone or soft man-to-man if the outside defender is giving a cushion.

It's a simple scheme, but the throw is not as easy as it looks. The quarterback must throw it accurately so the wide receiver can catch the football in full stride on his way toward the line of scrimmage. You actually may turn the outside defender loose (not blocking him) and just see if he can tackle the receiver in the open field.








Bubble screen vs. zone blitz
The bubble screen is also used to combat today's zone blitz schemes. If the defense blitzes and goes from a 2-deep hide to a 3-deep, it is a great play. Some offenses will actually abort a running play and throw a bubble screen if the linebacker blitzes. The only players who make this judgment are the wide receivers and quarterback. The offensive linemen and running backs actually go ahead and execute a running play. What the offense has done in the huddle is call two plays. In our system, we called this a 23 alert bubble (23 indicates a running play). The call allows the quarterback to audible to the bubble screen at the line of scrimmage.








The jailbreak screen
The jailbreak screen is a wide receiver screen that involves the wide receiver coming back to the quarterback at the snap of the ball. The reason it's called a jailbreak is because the offensive line releases automatically downfield to block. The offense uses a tight end or wide receiver to go away from the line of scrimmage to pick the outside receiver's man. The linemen punch the defensive line to stop their initial charge then release downfield to form a wall. The offensive tackle stays in and chops the defensive end to get his hands down so that the ball can be thrown over the top of him.








The conventional or slow screen
The least commonly used screen in today's football is the slow screen. This play requires a lot of timing and execution and involves a lot of deception. It is effective against zone defending teams and defensive linemen that get up the field causing separation. It is generally not effective against man-to-man as a linebacker immediately zeroes in on the running back.

The quarterback, instead of taking his traditional 5-step drop, actually drops deeper to allow the defensive linemen to rush up the field farther. This also allows the linebackers to drop deeper into coverage which creates separation. The offensive tackle to the side of the screen sets as if to pass block and then chops the defensive end -- once again so the ball can be thrown over him. The guards and centers hold for a one-thousand-one, one-thousand-two count and then release to form a wall -- usually to a landmark on the field. The halfback stays in, fakes pass protection and then slips out to be the receiver. As you can see, it takes much timing and execution, but if you catch the defensive in zone coverage, it can be an excellent play.








The crack screen
The crack screen is a tremendous play against man-to-man coverage when you know which defender is assigned to the running back. You simply crack a wide receiver from the outside in on the defender responsible for the running back. Once again, you are allowed to block back towards the ball as long as the ball is thrown behind the line of scrimmage and you don't block below the waist. In the example below, the wide receiver cracks back on the mike linebacker who is assigned to the back. As simple as this play looks, it is difficult for the man covering the wide receiver to switch off and take the mike linebacker's man.








Crack screen vs. man-to-man
In this particular play, the wide receiver cracks down on the linebacker who he knows is responsible for the running back in man-to-man. Again, you see the offensive tackle set and chop the defensive end and the center of guards release out in front of the pass receiver.








Summary
As you can see, there are a variety of screens to choose from. The success of the offense depends on calling the right screen against the defensive matchup. With the wide receiver screens, it is a simple execution play and a lot depends on the ability of the wide receiver to run after the catch. The big positive in screens is that they are basically simple to execute and provide you with the chance for a big play against an attacking defense.

Q & A with Bob Davie
First of all, thanks for all of the terrific responses and knowledgeable questions this week. I wish we had time to answer them all. Please keep sending in the questions and we'll tackle as many issues as we can this season. Here are a few of your questions regarding the zone blitz:

Coach,

The zone blitz packages seem to require a premium effort from strongside linebackers and the safety positions when it comes to reading a play, utilizing the speed to get there and then, sure tackling ability to boot.

Is this assessment correct? And if so, could you provide some examples of pro or college players who excel in these particular schemes?

Tom Puro
San Diego, Calif.

Bob Davie: I think it's obvious that good coaches find a way to do things with their difference making players and take advantage of their abilities. A lot depends on what type of players you have and putting them in a position to be successful. Good coaches design their schemes to take advantage of their best pass rushers and blitzers. Your assessment is correct. In the figures I used in last week's discussion of the zone blitz, I did highlight the strongside outside linebacker and the strong safety.

Watch the games every Saturday in college football and you will see that most defensive coaches will put their best defensive players in a position to wreak havoc on the opposing offense.

When I was at Texas A&M, we really took advantage of former quarterbacks and running backs, converting them to linebackers. Players like William Thomas, Quentin Coryatt, Aaron Wallace and John Roper gave us speed at outside linebacker and allowed us to blitz effectively.



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Hey coach,

Unless it is Julius Peppers, isn't it asking a lot of the defensive end to cover a 1/4 to a 1/3 of the under coverage? Isn't that where an offense should go to exploit the weakness in a zone blitz?

What's to stop the X from running off the corner with a go and then sending H on a flat route or a wheel up the sideline? Most defensive ends shouldn't be able to run with a running back.

I can see the zone blitz being effective as a changeup, but if a defense relies on it too heavily, I would think it creates a pretty big mismatch for a smart offensive coordinator.

How often did you employ the zone blitz on passing downs?

Tim
Tumwater, Wash.

Bob Davie: I think you bring up a tremendous point. This week's column on screens shows a great way to combat the zone blitz. It's also great to throw in the zone where a defensive lineman is assigned in coverage. One positive of the zone blitz, though, is that the ball has to come out so fast. Normally, the offense doesn't get a pre-snap read that the zone blitz is coming. The defense depends on that surprise element.

Offenses have evolved -- everything does run in cycles -- and are starting to do a better job against the zone blitz. As a result, you're starting to see less and less of it.



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Great column. This is just about the best thing espn.com has ever done, in my opinion.

I'm wondering, with regard to the zone blitzes, under what situations a defense would opt to use a given blitz. They all kind of look similar to me, but would there be certain game situations that a defense would blitz a safety as opposed to a backer?

Also, when would a defense use a 3-deep zone as opposed to a 2-deep?

Bruce Hansen
Orem, Utah

Bob Davie: Actually what happens is that the zone defense scheme has evolved to where defenses will check from one zone blitz to another depending on the offensive formation and tendencies. There are definitely more zone blitzes that are more advantageous for certain formations and against runs or passes. The defense tries to check into those. You are right, with every zone blitz you call, there is a tradeoff. What you try to do is check into the zone blitz that gives you the best opportunity against the formation the offense is in or what play you think they will run.



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Coach Davie,

Just curious, in your version of "SLB zone blitz", you have a tackle dropping back to cover in the middle for a linebacker. Do you expect him to be effective in this or do you just get a big body in there that can disrupt the flow in his zone? And if it is a pass play, can he really get back there fast enough to make a difference? Thanks

Nashvol
Nashville, Tenn.

Bob Davie: The biggest reason that you would drop the tackle into coverage would be in case it is some kind of screen pass and you just don't want to give up a big chunk of the field. We actually did that zone blitz with a 3-underneath, 3-deep concept many times -- particularly on a running down. We would not ask the nose guard to have any pass responsibility at all. If you are calling that on a passing down, instead of just voiding the middle of the field, you may ask the nose guard to drop out late. If it's a running down, you probably void it completely.

Over the past couple of weeks we have had tremendous feedback about the Football 101 column. What we will try to do in the following weeks is answer your most frequent questions. Many of you have had questions in regard to defensive pass coverages and specifically Cover 2. Every defensive football team in the country has some variation of Cover 2 in their defensive package.

This week's class will explain what Cover 2 is, how to play it and some variations. We will also show ways offenses can attack it.

Terminology
Why is it called Cover 2
When deciding the terminology of calling coverages, the number of deep zone pass defenders that are deployed will normally determine what a defensive coach calls a defense. In Cover 2 for example, there are two deep safeties that divide the field into halves. If the secondary played Cover 3, three deep defenders would divide the deep responsibility on the field into thirds. If they played Cover 4, four deep defenders divide the deep zone into fourths.

Obviously, different teams use different terminology, but the most commonly used is simply identifying how many deep zone defenders are used.

What is Cover 2?
The base Cover 2 is a zone defense where every defender is responsible for an area of the field and not a specific man. The field is divided into five underneath zones and two deep zones. The two corners and three linebackers play the underneath fifths, and the two safeties play the deep halves. In the diagram below you can see the field divided into underneath fifths and deep halves.








Cover 2: How is it played?
In Cover 2, it is obvious that the safeties have a tremendous burden and a lot of field to cover. They must get help from the underneath coverage to keep receivers from outnumbering them in the deep zones. There are two critical techniques that can help the safeties. First, the corners must collide with the receivers and flatten out their routes to keep them from running outside freely, which would stretch the safeties. If the wide receivers release unmolested, it is almost impossible for the safeties to get enough width quickly enough to defend the deep pass. The corners are responsible for their outside fifths, which is a shallow area, but they must sink with the receiver until another receiver threatens their zone.








Pattern read: What is it?
As you can see, Cover 2 is a zone coverage, but it is critical that the underneath defenders at times play man-to-man based on the routes the offense runs. This is the second technique that helps the safety. For example, if the wide receiver to the linebacker side releases outside in a position to stretch the safety deep, the linebacker must know that. He then must carry his coverage on the running back or tight end through the deep zone. This is called pattern read, which means based on what pattern the offense runs, man-to-man principles are applied. This is an example of a linebacker making a pattern read. In the diagram, he sees that the receiver (1) releases outside the corner and he knows that he must carry (2) through the seam where the safety will be outnumbered.








Cover 2: How offenses attack it
(1) Four vertical receivers
The first thing an offense will do if they see Cover 2 is see how you defend four vertical receivers running down the field stretching the two safeties. If Cover 2 is not played properly, this is the first place the offense will go and it could result in a big play. In the previous figure, we showed how to defend four vertical by the corners flattening out the wide receivers and the underneath coverage linebackers running with the vertical of the tight end and running back. But it is also obvious by the previous diagram that if the linebackers fail, a big play is possible.

Cover 2: How offenses attack it
(2) High-low stretch vertically on the corner
The next way the offense will attack Cover 2 is to stretch the corners vertically. The most common way to attack Cover 2 is to high-low the corner who has the outside fifth or flat responsibility. The outside wide receiver runs a post-corner route. In other words, he fakes as if he will run a deep route, which freezes the safety, then breaks back outside to the corner. He knows the safety must stay inside and protect his inside one-half and not allow the receiver to cross his face (which means getting inside of him). The corner tries to sink and help defend the hole between he and the safety to his outside. The offense will then release a tight end or running back into the corner's fifth to create a high-low stretch.








Cover 2: How offenses attack it
(3) In-Out stretch horizontally on the corner
Another way to defeat cover two is to horizontally stretch the corner and create a void between the corner and either the SAM or WIL linebacker. The offense does this by outside releasing the wide receiver to make the corner flatten out and widen. As we stated before, the corner must jam and widen to keep the receiver from releasing outside and stretching his safety. The offense takes advantage of the corner's technique by making him widen with the wide receiver's release and then sending the No. 2 receiver either tight end or running back into the void or stretched area between the linebacker or corner.








Variations of Cover 2
Man under
The most common variation of cover two is when the defense chooses to play two deep safeties but assign the five underneath defenders to play man-to-man on the offenses five eligible receivers. This is an excellent change up because it looks exactly the same as regular cover two zone to the offense. On the snap of the ball, the offense is confused because they have called their zone beaters but the defense locks up on the man-to-man receivers.








Conclusion
Cover two is an effective coverage because you can assign five defenders to play the underneath zones. This makes the offense have to execute at an extremely high level to be effective. The defense is not allowing much space or easy throws. The vulnerability of cover two is that you only have two deep defenders. Obviously, that leaves a large area for the safeties to cover. To help the deep defenders, the underneath zone players must pattern read the routes of the offensive players to take the pressure off the safeties. The key to cover two, or any zone coverage, is getting a great pass rush with your four rushers. No zone coverage can hold up against a good passing football team if you don't pressure the quarterback.

Q & A with Bob Davie
First of all, thanks for all of the terrific responses and knowledgeable questions this week. Please keep sending in the questions and we'll tackle as many issues as we can this season. Here are a few of your questions regarding the screen pass:

Coach,

Very informative column on the screen pass. I have a question for you, though. What defenses are most adept at stopping the screen? I'm an Oklahoma fan, and I've seen a lot of screens blown up in the last couple of years. I am wondering if the offensive players are missing assignments or if the defensive scheme is killing the screen before it gets started?
Bryan
Oklahoma City, Okla.

Bob Davie: The key is to call the right screen against the right defensive coverage. The slow developing delay screen, thrown primarily to the backs, is predicated on the defense being in zone pass coverage and taking deep pass drops. Another example, the jailbreak screen, is more effective against an all-out blitz or man-to-man coverage where there is separation between the rushers getting up the field and being able to pick defensive backs in coverage. Just like anything in football, it's all about the right call against the right coverage.



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While well-executed screens can be great plays and are essesntial to keep a blitzing team honest at all times, I hate watching games where an offense calls the same 1 or 2 screens 10 times each. Perhaps even worse than the redundant play calling, often defenses are still getting burned by the same play the 8th time it is called. What are offenses doing to disguise screens? What elements of screen plays make them more difficult for defenses to adjust to?
Brad Wilks
Athens, Ga.

Bob Davie: I don't think you can really disguise a screen. But what you try to do is through your own self scouting, not call screens on predictable downs and distances. Give defenses teams credit, they study your tendencies and it's imperative that whether it's screens or just another play that you don't get in a pattern of calling similar plays in similar downs and distances.



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Coach, keep up the good work. About your column on screens, one effective type is the fake-throwback screen where two or three linemen pull off to the right side and the QB fakes a screen to that side and then wheels and throws to a back slipping out the other way. I have seen this work for big plays. Why don't more teams try this at least once a game?
Seve Jordan
Wayne, Pa.

Bob Davie: I think you bring up a good point -- and it's called the double screen where you actually set up the screen to both sides. I think you're right that you don't see that called as much anymore because it's a high, high execution play. The other thing you don't see called and Florida State used to be good at it, set up the double screen to both sides, get the defense spread out and then delay the tight end and get him the ball. It's another good play.

The key to good offensive football is having a balanced attack with equal amounts of running and passing. With the emphasis on the passing game and the evolution of 8-man fronts and zone blitzing defenses, it has become more and more difficult to be able to effectively run the football. Over the past few weeks, we have had a lot of questions regarding zone blocking and exactly what that means. In this week's class we will try to explain zone blocking by the offense and why it has become a common scheme used to run the football.

What is zone blocking?


Zone blocking in the running game is when two or three offensive linemen work in tandem as opposed to each offensive lineman having a specific, predetermined man to block. Zone blocking involves the center, guard, tackle and tight end working in combination to block an area with an emphasis on double-teaming the defensive linemen who are aligned on the line of scrimmage.

The concept is for two adjacent linemen to come off in unison and attack a defensive line to the play side or to the side the ball carrier is going. The advantage, as opposed to man blocking, is that you create a double-team with two players blocking one defensive lineman. This allows the offensive linemen to be aggressive because he knows he has help if his defensive lineman was to pinch inside. It also provides movement at the point of attack, which can open creases for the running back.

Zone blocking initially starts out as a double team at the point of attack on the down defensive linemen, but the beauty of it is that one of the offensive linemen will leave to attack the linebacker while one stays to take over the defensive lineman. The key is for the two offensive linemen working in unison to double-team the defensive lineman to decide who and when one of them will leave to block the linebacker. In the diagram below, we show the offensive line starting the initial double team on the defensive lineman.








It appears that they have doubled the defensive end and defensive tackle and allowed the linebackers to go free, but both offensive linemen on the double team have all four of their eyes on the linebacker while the double team is taking place. One of the linemen will come off the double and block the linebacker.








There are several keys to this technique:
1. The linemen stay hip to hip.
2. The linemen keep their shoulders square.
3. Most importantly, all four of their eyes are on the linebacker.
4. Knowing when and who takes over the defensive lineman and who leaves to block the linebacker.

In the next diagrams, we show the technique of going from the double team to taking over the linebacker. If the end pinches inside, the guard will take over and the tackle will leave for the linebacker. (See below)








If the end stays outside, the tackle will take over and the guard will leave for the linebacker. (See below)








Difference between man blocking and zone blocking


Zone blocking first started to take place back when teams ran an old slant and angle defense. They would line head-up on an offensive lineman then slant the defense one way or another. It is easy to show this problem in man blocking and the best way to illustrate it is to show the defensive end pinching inside. If you are in man blocking and the tackle is assigned to the defensive end, he not only misses the defensive end pinching, but the DE knocks off the guard and keeps him from going to the linebacker.








There are different kinds of zone plays and you will often here the term the outside zone. In the figure below, we show the landmark of the back in the outside zone. It is obvious that at the angle the back takes the ball there is very little opportunity for the back to cut back behind the center. This affects all of the linemen's techniques because it is predetermined where the ball is going.








The inside zone is another term you hear. On the inside zone, the back's angle is more to the inside leg of the offensive tackle. Because the back is headed in a more straight-ahead angle, there is now the ability for the back to cut back behind the center. It is important that the back gets into the heels of the offensive linemen before he makes the cut. The offensive line can't allow penetration.

Pass blocking
Zone or Man
Zone-locking or man-locking principles may also be applied to pass blocking. Offensive linemen, when facing twisting defensive linemen, can also either lock on man-to-man or pass it off in a zone concept. When passing it off, or zone blocking, the key is to stop the penetration of the defensive end. In the figure below, the offensive tackle must stop the penetrating defensive end before passing him off to the guard. The offensive tackle then takes the defensive tackle looping around.








Conclusion
Zone blocking was created to handle moving defensive linemen. It is a simple concept, but it takes a lot of practice because it involves offensive linemen working in unison and decisions have to be made while the play is taking place. In zone blocking, you don't have a lot of different assignments, but you have a lot of techniques. It takes many repetitions to get the feel of working together as a unit. The diversity of zone blocking comes by the back running different angles and by the offense using different formations to confuse the defense.

Q & A with Bob Davie
First of all, thanks for all of the terrific responses and knowledgeable questions this week. Please keep sending in the questions and we'll tackle as many issues as we can this season. Here are a few of your questions regarding Cover 2:

Coach,

Which college teams this year are devising the best attack schemes to defeat the Cover 2 zone coverage and could you give some examples from some of the "September" games? Thanks!
Brian Burnett
Colorado Springs, Colo.

Bob Davie: I don't have a specific example from this year, but I think there are a couple of concepts. The first depends on having a great tight end, like Miami last year. In this case, you try to get the tight end up the seam on the linebacker. Most teams attack Cover 2 the same way. Something I didn't mention is that when a defense deploys four defenders in Cover 2, that leaves a 7-man front. Teams will try to run the ball against it. There is not a specific team that comes to mind, but Miami with Jeremy Shockey comes to mind.



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Coach,

Can a team that uses a base 3-4 defense use Cover 2? If so, how does it differ?
Reed David
Pittsburgh, Pa.

Bob Davie: Definitely. They have a few options too. First, defenses can use one of the outside or inside linebackers as the fourth rusher. You would still play a 5-under, 2-deep concept with a 4-man rush, but one of your linebackers becomes the fourth rusher. Another way a defense uses Cover 2 from a 3-4 base is to just rush three defenders and drop all four linebackers, with the two corners and play 6-under, 2-deep and have an extra underneath dropper as a "rat," who looks for crossing routes or quarterback scramble.



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Thanks for explaining Cover 2, Coach Davie. I have one follow-up question: what happens with slot receivers? Seems like that's an immediate way to beat a zone, to put 2 or even 3 receivers on one side of the field. Does the D then have to audible to man-to-man? Many thanks--I'm learning a lot!

Derek Oja
Iowa City, Iowa

Bob Davie: No, you can play Cover 2 against any formation. The difficult thing with the slot is many times you get a linebacker that has to "walk out" particularly in a one-back set on the slot. That's how the evolution of nickel defenses took place. Teams will substitute a fifth DB for a linebacker to be able to play either zone or man-to-man coverage on the slot.



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I appreciated you explanations of and insights into Cover 2. You mentioned that it is often important for a LB to read the WR's pattern, specifically if the WR had an outside release. You wrote that an outside release indicates that WR might be going deep and that the LB should play any inside receivers (RB, TE), man-to-man in case the offense is trying to stretch the safeties w/ 4 deep receivers. If a LB must read run first, how can he also read the WR releasing outside? It seems like too much to read and react to at the same time. GO DAWGS!
Graydon Simkins
Tacoma, Wash.

Bob Davie: Great question! The outside linebacker always, always plays run first. The tight end will tell him if it's run or pass. If the TE blocks, the LB is obviously playing run. The only time he reads outside to the WR is if the TE releases pass. The LB should not kick into a pass read until the TE declares that it is a pass