Gaps in coverage are a major determining factor of a team's success on offense or defense. Offensive players seek to fill gaps when defenders attack and open up alternative gaps at the line or downfield to help free ball carriers and eligible receivers. On a defensive unit, players are assigned different roles in a given scheme to plug these gaps, close on ball carriers quickly and minimize gains of yardage. How these gaps are covered can vary depending on the goals of the defensive unit on a given play.
Gap Types
Two basic gap types exist in football: coverage and line of scrimmage. Coverage gaps consist of spaces or zones on the field where a receiver has a good chance of catching a pass before the defense can close in on him. These gaps are created in different ways depending on whether the defense is playing man defense or zone coverage. Running gaps are important at the line of scrimmage and are created by the offensive line and other blockers. Gaps not covered by the defense are options for a ball carrier to run through and gain yards.
Defensive Line
The defensive line gap responsibilities are important for stopping a running game. If ball carriers can consistently make it to the second line of defense -- that is, linebackers and secondary players behind the defensive line -- it will be much harder to stop an offense and keep it to a low point total. Defensive linemen are expected to attack the gaps between offensive lineman to help close running lanes for ball carriers and crimp the offense's ability to run the ball effectively. If a defensive lineman can break through the offensive blockers, it could produce a tackle for a loss of yards or a sack of the quarterback.
Secondary
The gap assignments for secondary players are different than for those at the line. Secondary defenders are primarily concerned with stopping the pass. The gaps that must be covered are spots on the field where a quarterback might try to complete a pass to a receiver. These gaps are typically covered in one of two ways: Xecondary players match up directly with eligible receivers and attempt to stick with them and be present to stop a pass from being completed; or, the secondary may use a zone to cover different sections of the field where pass attempts are likely to occur. It is also possible for secondary units to use a mix of man and zone coverage. These coverages often are changed from play to play to keep the offense confused.
Linebackers
Linebackers have the most versatile possibilities -- they can move up and perform roles similar to defensive linemen or they can drop into coverage and defend against the pass. Linebackers are brought in between defensive lineman to overload the offensive line, making it possible for linemen or linebackers to attack unguarded gaps and enter the backfield. Linebackers also may serve as "spies," a term used when one player is designated to keep his eyes on a backfield player -- often the quarterback -- and pursue that player if he leaves the pocket.
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