Introduction
In Gameplan the defensive play calls are a combination of formation (the players on the field and the positions in which they set up) and action (what they do at the snap of the ball). The exact combinations of defensive personnel, their alignments and assignments are never actually specified (this isn't possible because of the scale of the mismatches that can occur in the game). The defense may sometimes be assumed to being doing something different from what you ordered, if it is obvious that it should do so. For example, if you call a goal line defense against a deep pass from shotgun, then your defenders do not simply stand around on the line of scrimmage waiting for the run. They read the pass and adjust to the call. If, on the other hand, you were to call a goal line defense against a play action pass from a run formation they wouldn't adjust at all and you'll be lucky if you don't give up a touchdown. The table below shows the play call codes, names and general description of each of the defenses available, along with the number of men initially committed to the pass rush.

 

Base Formations
There are currently two defensive "base" formations, the 3-4 and the 4-3. The choice between which of these to use as your base defense is considered part of the rosters rules (see Rosters Guide).
The difference between the two is mainly that moving from the 3-4 to the 4-3 one of the linebackers is replaced by a defensive tackle. This improves the straight ahead run defense and the initial pass rush (because of the extra lineman) at the expense of the outside run coverage and short pass defense (one less coverage defender). The 4-3 is usually less flexible, as there is a greater degree of specialisation. Both formations hinge on a single key player who must dominate the middle of the formation (the nose tackle in the 3-4, and the middle linebacker in the 4-3).

 

Pass Rush
Regardless of the defensive formation used, most pass defenses have a basic four man pass rush. For the 4-3 formation these four rushers are simply the four defensive linemen. For the 3-4 defense the three defensive linemen are usually joined by a linebacker, often a player actually referred to as "designated pass rusher". This is usually the (blindside) right outside linebacker, who on passing downs often lines up on the line of scrimmage outside the defensive end. This blurs the distinction between the 3-4 and 4-3, as the remaining three linebackers responsibilities are the same as they would be in a normal 4-3 set. In Gameplan any single dogging linebacker in 3-4 counts as a pass rusher in all defenses, and he is not considered to be a blitzer.

 

Run & Pass Defense
In any formation the defensive players either play for the run, by trying to prevent the offensive line from opening holes through the line of scrimmage for the running backs, or play for the pass, with pass rushers trying to break through the offensive line and pressure the quarterback before the receivers can get open. Pass defenses are characterised by defensive players trying to penetrate into the backfield whilst the linebackers and defensive backs cover potential pass receivers. Normally the secondary are responsible for covering the primary receivers, with the two cornerbacks covering the two wide receivers, the strong safety covering the tight end, and the free safety patrolling deep in centre-field as a last line of defense, with the linebackers having responsibility for any running backs. No defender can keep a top class wide receiver covered for ever, and any passing down is a race between the receivers getting open and the pass rushers reaching the quarterback.
Run defenses are characterised by the defense not trying to penetrate the line of scrimmage, but trying to ensure that their strength remains focused around the ball. The secondary will still provide coverage against the pass, but with a weakened pass rush their chances of keeping all receivers covered for long enough are very much reduced.

 

Basic Defenses
The more you specialise, the greater the chance of getting a serious mismatch. Some defenses are very extreme (very good when they're good, and very bad when they're bad). You can gamble against virtually any offensive play call, but you're not obliged to gamble (unless your defense can't do the job any other way).
Run Defense (RD) is a conservative run defense, with the linemen and linebackers looking to plug the gaps and not allow the offence more than a two or three yard gain. There is no initial pass rush, but the defense is not very strongly committed to the run and is unlikely to give up a big gain to a passing play.
Pass Defense (PD) is a four man pass rush with man to man coverage on the various receivers. The outside linebackers hold responsibility for the running backs, the cornerbacks cover the wide receivers, and the strong safety is responsible for the tight end. The corners usually line up fairly deep, giving them time to react to the receivers' moves, and the overall passing threat should be reduced. The defense is not particularly weak against the run, as the defenders are individually keyed on the various offensive personnel (for example, if the tight end blocks rather than releasing, the strong safety should read this and move up for the run).
Mixed Defense (MD) is a standard defense, with the front seven reading and reacting to the offence. The defense is unlikely to make a big play, as they're conceding the initiative, but they're unlikely to give up a big play either. In many combinations mixed defense is also treated as a specialist defense. If the offence is messing about with something clever, then mixed defense will help your defense to wait, read and react correctly.
Goal Line (GL) defense defends the line of scrimmage, stacking up and pursuing the play along the line. It is good against most running plays, effectively keying out the running backs, although liable to be penetrated by dive plays (where the defense is not concentrated enough to hold the line) and misdirection plays (where the aggressive pursuit can be drawn to the wrong point of attack). Against the run it is not likely to give up big gains. Against the pass it is poor, with the front seven committed elsewhere and no pass-rush, making the chances of a completion and good yardage high, although each individual receiver should still be checked off in man to man coverage.

 

Run Defenses
There are a number of other run defenses, some of which are variations of the basic run defenses, while others are more specialised. Many teams will adopt one or more of these defenses as a basic defense, and you will often find it necessary to adapt your defense to use one or more specialist run defenses when facing a powerful run offence.
In the Overshift (OV) defense the defensive line overloads the strongside of the line of scrimmage by slanting their blocking assignments to that side. Against the pass there is no difference from a basic run defense, but against strongside runs, particularly off-tackle plays, the extra concentration of the defensive line is effective in increasing the number of busted plays (stuffs). Against weakside runs, the defense is less effective, with the defense slanting away from the point of attack.
The Undershift (UN) defense is the mirror of the overshift, with the defensive line slanting the weakside of the line of scrimmage. This isn't an especially good idea unless the offensive is running to that side. Look out for offences with balanced strong and weak sides, who are especially likely to hit the weak side of the line.
The Flex Defense (FD) is a more aggressive run defense with the defensive linemen attacking across the line of scrimmage to break up the play, while the linebackers read and react as in the basic run defense. The linemen's charge increases the chance of stuffing a play at source, and the play is strong against inside runs, which are likely to be disrupted by the rush or plugged by the linebackers. With the defensive line committed to the backfield there is a risk if the ball is run to the outside, but the play is flexible if the play turns out to be a pass. The hunt for the ball in the backfield will soon turn to search for the quarterback if he drops back to pass.
The Tackle Stunt (TS) is also an aggressive run defense with the line stunting to spring a man inside. In a stunt one lineman leaves his position and loops around another lineman into what might be a vacant hole. This increases the chance of a lineman being sprung unblocked into the backfield, but also increases the chance of a breakout, if the position vacated is the hole to which the offence is running. The play is most effective against the slower developing runs, particularly lead plays.

 

Pass Defenses
All pass defenses are a balance between keeping receivers covered and reducing the time they need to be covered. Given time any receiver will get open eventually. There are two forms of pass coverage, man to man and zone. In man to man coverage each defensive player has responsibility for covering a specific opponent, but in a zone defense the defensive players cover specific areas of the field (known as zones). Instead of running with the pass receivers, the players remain in their zones until the ball is thrown and they are free to converge upon the target. Zone defenses and defenses with spare men in pass defense are much more likely to gain interceptions and don't have the same risk of long gains as aggressive defenses. They will also generally give up less primary completions and more dumpoffs, and will be more vulnerable to draw plays.
The Bump and Run (ND) defense is a variation of the basic pass defense. The cornerbacks line up on the line of scrimmage and try to "bump" the receivers, preventing them from getting away into open field (defenders are only allowed this sort of contact with receivers close to the line of scrimmage). This defense is also effective against quick patterns as the receiver cannot catch a pass "underneath" the coverage if the defense is tight to him at the line of scrimmage. However, the aggressive play of the cornerbacks increases the chance of a "blown play" and a big gain if a pass is completed.
The End Stunt (ES) is another variation of the basic pass defense. Instead of a straight ahead four man rush, one pass rusher stunts (loops around another defensive lineman) to the outside. The disruption to the offensive blocking scheme increase the chance of springing him free with a clear path to the quarterback. Against a long pass (slower pattern) the chance of disrupting the play should be higher, against a quicker pattern the stunt may simply take a pass rusher out of the play.
The Double Man Defense (DD) is a more extreme pass defense, dropping a man from the pass rush to reinforce the coverage. This provides either double coverage on a key receiver or an extra man deep. With only a three man pass rush, the chance of quickly closing down the play is poor, but the extra defender decreases the chance of giving up a big completion and increases the chance of making an interception.
The Zone Defense (ZD) is a committed pass defense. The front four rush the passer, and the coverage is divided into seven zones, with four short zones up to 15 yards from the line of scrimmage, and three deep zones further downfield. With the secondary concentrating on the quarterback, they cannot react quickly to running plays or screens. Zones can also be vulnerable to quick patterns, when a receiver may be isolated against a linebacker, and to secondary receivers slipping into the "seams" while the defenders are picking up primary receivers tracking through their zones.
The Short Zone (ZS) defense emphasises the short zone coverage, with five short zones and only two deep (hence this is also called the "two deep zone"). The two outside short zones are both manned by cornerbacks, so it is particularly strong against short patterns run near the sideline. The two deep zones are manned by the two safeties, who may be vulnerable to any deep pattern, but particularly to a fast tight end breaking up the middle into the seam between the two safeties.
Wide Coverage (WC) is primarily a zone defense concentrating on avoiding giving up the big play. There are four short zones (manned by linebackers, except that a defense would normally replace some linebackers with defensive backs in situations where wide coverage might be used) and four deep zones (all manned by defensive backs), and only a three man pass rush. Entirely useless against a running play, as the defense concentrates on pass coverage. Close to the goal line, where the deep zones are unnecessary, the defense adjusts by keeping pass defenders in man to man coverage to ensure that on any chosen play wide coverage always has the least chance of giving up a catch in the endzone.
The Linebacker Drop (LD) is a mixture of man-to-man and zone defense. One pass rusher drops off the line of scrimmage at the snap of the ball into a short zone over the middle, from where he can read the quarterback and/or pick up running backs and tight ends coming over the middle. If the offence is passing deep or out to the wide receivers then the pass rush is reduced without reinforcing the secondary, but a key feature is the opportunity for the offence to lose track of the dropping linebacker or lineman and throw a free ball into his zone.
The Linebacker Jam (LJ) is a variation on the basic pass defense. The strongside linebacker attempts to jam the tight end on the line of scrimmage, preventing him releasing into his pattern before passing coverage over to the strong safety. Effective against passes to the tight end (who is effectively being double covered) but not good if the play calls for the tight end to block the linebacker anyway, and also removes the linebacker from pass coverage in the flat.

 

Blitzes
Blitzing shifts the balance of the defense by taking players out of the run or pass coverage, risking longer gains, but gaining the chance of nailing the play at source. A blitz can stuff a run just as effectively as a pass, but probably not unless you direct the blitzer the same way as the ball carrier. A "blitz" actually means sending an extra player from the defensive secondary, while an extra pass rushing linebacker would be "dogging", but "blitzing" is often used to refer to either. Taking a man out of the coverage is always a gamble, and deciding which man to send is an equally difficult choice. Weakside blitzes generally have more chance of reaching the quarterback (being on his blindside).
The Strongside Linebacker Blitz (JB) sends the strongside (left) outside linebacker looping to the outside. His chances of disrupting a sweep are good, as he may get into the backfield and nail the running back, and the five man pass rush also provides a strong rush against the pass.
The Weakside Linebacker Blitz (KB) is a similar play, but sends the weakside outside linebacker looping to the outside (in a 3-4 defense it is the weak inside linebacker who blitzes, having assumed the responsibilities of the outside linebacker who is already the fourth pass-rusher). The effects are similar to the strongside, except that weakside runs may be stuffed.
The Inside Linebacker Blitz (LB) is the least effective blitz against the pass, sending a linebacker through the middle of the offensive line. Against a pass the linebacker is unlikely to get through unblocked, but against the run he increases the chance of stuffing a play up the middle.
The Free Safety Blitz (FS) is a disguised or delayed blitz up the middle by the free safety. The relatively light but quick free safety is unlikely to be very effective at plugging a gap on a running play, but against the pass he's the least likely to be picked up. He was also the last line of defense, until you sent him after the quarterback.
The Strong Safety Blitz (SB) is similar to the Inside Linebacker Blitz, sending the strong safety up the middle. The strong safety will be less effective against the run than a linebacker, but more effective than the free safety, and is more likely than a linebacker to be left unblocked on a passing play.
The Strongside Corner Blitz (SS) is actually a blitz by either the strongside cornerback or the strong safety from the outside. Less effective against the runs than linebacker blitzes, the element of surprise against the pass will be higher, as the blitzer is leaving his coverage on a primary receiver.
The Weakside Corner Blitz (WS) is similar to the strongside blitz, but with the weak (free) safety or weak side cornerback attacking from the outside.

 

Death Or Glory Defense
Sometimes, when the defense knows for sure what the offence is going to do, and playing the percentages isn't good enough, then there comes a time to gamble hard. All three defenses offer a high chance of breaking up any play in the backfield (particularly those they're supposed to stop). The problem comes when they don't get there, everyone has been committed and you should expect to give up big yardage if you don't kill the play at source.
The All Out Blitz (BZ) combines the weak side linebacker blitz and the strong safety blitz, with both men rushing the quarterback to make up a six man pass rush. The chance of nailing the quarterback is very high, but if the ball escapes the remnants of the defense will likely be out-matched.
The Inside Charge (IX) is the most extreme inside run defense, with the front six stacking up the play inside. Any inside run will be overwhelmed, but against an outside run or any pass the only chance is to stuff the play at source. Normally only used on goal-line or short yardage situations.
The Box Defense (OX) is an equally aggressive defense, that commits defenders to the outside to box running plays to the inside. Outside runs will be stuffed, but if a running play breaks inside the yardage will be significant. Against a pass the only hope is the pass rush.