american football defensive schemes

In order for coaches and players of American football (and, with the necessary modifications for adding a twelfth player, Canadian Football) to exchange information in a rapid manner during practices and games, a more or less standard terminology for defensive schemes has been developed.

Basic alignments

Players on the defensive side of the ball are generally split between down linemen (tackles, defensive ends and nose guards), linebackers, and defensive backs (safeties and cornerbacks). To describe the basic defensive allighment of linemen, linebackers and backs, the number of down linemen is usually followed by the number of linebackers. By far the most common alignments are four down linemen and three linebackers (4-3), but alignments with three down linemen and four linebackers (3-4) are currently used by a number of teams. The number of defensive backs is usually not mentioned (as it is, for example, in describing soccer alignments). However, on plays where the defense expects the offense to pass, emphasis is often placed on the number of defensive backs. When one of the "front seven" (down linemen and linebackers) is removed in favour of a defensive back, the five defensive backs are described as a "nickel" package. When a sixth defensive back is inserted, it is known as a "dime" package. Unusual defensive alignments are rare, but often successful. In Super Bowl XXV, the New York Giants played with only two down linemen, with four linebackers and five defensive backs. The strategy was very successful in preventing the Buffalo Bills from completing long passes, but it allowed over 190 yards in rushing. Nevertheless, the Giants won.

Basic pass coverage

Even in obvious running situations, the defense must be able to account for the eligible receivers on offense. There are two general schemes for defending against the pass:

Man-to-man, where each eligible receiver is covered by a defensive back or a linebacker.
Zone, where each linebacker and defensive back is assigned an area on the field which they are to cover.

In most cases, man-to-man coverage is more effective against short passes, while zone coverage is effective against long passes.

Advanced pass coverage

To create a shorthand, most defensive schemes use the term "cover" (for pass coverage) and a number to describe a combination of schemes. As in American Football there are only five eligible pass receivers on a given play (technically the quarterback is also an eligible receiver, but passes to the quarterback, though known, are rare) while there are at least seven pass defenders in 3-4 alignment in man-to-man defence, some of the pass coverage personnel may either blitz (cross the line of scrimmage with the down linemen in an attempt to sack the quarterback), provide double coverage on a receiver, or help other defensive players with the pass coverage. In zone coverage, all defensive linebackers and backs have a pass coverage assignment.

Cover Zero - Strict man-to-man coverage with no help from free safeties (usually a blitz play with at least five men crossing the line of scrimmage)
Cover One - Man-to-man coverage with at least one safety not assigned a player to cover who can help out on deep pass routes.
Cover Two - Zone coverage with the safeties playing deep and covering half the field each.
Cover Three - Zone coverage as above, only with extra help from a cornerback, so that each player covers one-third of a deep zone.
Cover Four - As above, with the corners and safeties dropping into deep coverage, with each taking one-fourth of the width of the field.

Generally speaking, the effectiveness of a defense against short passes and the run drops as it goes from Cover Zero to Cover Four, but their effectiveness against deep passes increases.

Strategy

Effective defence depends on co-operation from defensive players and an understanding of what coverage they are in. For example, in Cover Two, the cornerbacks must be careful to jam the wide receivers at the line of scrimmage so that they cannot run deep too quickly. Moreover, if the receivers do break deep, the cornerback must drop back to ensure that the safeties are not spread too wide, leaving a hole between them. In Cover One, the safety must be aware that one of the cornerbacks could have difficulty covering a wide receiver, and must be available to move over to help the cornerback before the quarterback can throw. Moreover, mixing up defensive alignments, and not being predictable, are important as if an offense recognizes an alignment or coverage scheme, or a tendency to use such a scheme, they can often take advantage of it. For example, if the defense is blitzing, and the quarterback forsees it (for example, one of the blitzing players moves towards the line of scrimmage before the snap) the quarterback knows that it is man-to-man coverage and will look for his fastest receiver to get open, or throw to the spot that is vacated by the blitzing player.

Special Cases

In the modern game, with players getting faster and stronger, defensive co-ordinators often look to a player's special skills in order to surprise the offense. For example, in some defensive schemes, defensive down linemen are given pass coverage responsibility. Since Lawrence Taylor demonstrated the effect of a single blitzing linebacker on an offensive line's blocking scheme, most teams now rush three down linemen and a single linebacker (often a different one on every play), a strategy that was almost unknown before he started to play. Moreover, even defensive backs are being given more responsibility on running plays. For example, on plays where a running back runs wide, it is the responsibility of the cornerback to ensure that the running back does not get directly to the sideline, and that the back is forced to run in front of the cornerback where there is more likely to be help from linebackers. Modern offences have adapted to these strategies, and often require different skills from players, particularly running backs who, in addition to running with the ball, are expected to run deep pass routes against linebacker coverage, and to be available to block blitzing players on pass plays.