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The prevent defense is a defensive alignment in American football that seeks to prevent the offense from completing a long pass or scoring a touchdown in a single play and seeks to run out the clock. It is used by a defense that is winning by more than a touchdown, late in the fourth quarter, or in specific situations, such as third-and-very-long if it seems clear that the offense must pass the football to gain long yardage.
The alignment uses five or more defensive backs (or players in that role), preferring fast players over large players. They back up so far that they concede short-yardage plays but try to ensure that no receiver is uncovered downfield or can get behind them.
The prevent defense aims to prevent a big play, like a 25-yard or longer pass or run. The defense concedes short gains, such as four to eight yards per play, as long as the clock keeps running.
Safeties and cornerbacks all pull back to a "safe zone" five to ten yards off the line of scrimmage, and the free safety often plays as far as 20 yards back. The defense does not jam receivers on the line. Part of the meaning of "safe zone" is that the prevent defense uses zone defense in which each defensive back is responsible for an area of the field rather than a specific player. The backs watch the quarterback's eyes to determine where he intends to pass the ball.
When used late in the fourth quarter to run out the clock, a goal of the prevent defense is to keep the ball carrier in bounds rather than to let him cross a sideline, which stops the clock.
The prevent defense uses five or more defensive backs. The nickel defense has five backs, so named because the nickel is the five-cent coin. Defenses with additional backs take the name of the next coin, but numerically, they are misnomers so the dime defense has six backs.
- Quarter defense
Quarter(s) can also describe a type of zone pass coverage in which four defensive backs divide the field into four vertical zones. This coverage may be combined with a 4-3 or 3-4 and is used to take away deep routes but maintain a good pass rush and run coverage.
- Half dollar defense
The half dollar defense has eight defensive backs. The rare package is used when the offense needs to score a touchdown on the very next play, such as with a desperation Hail Mary pass.
Professional teams may not have enough defensive backs on the roster to play the quarter or half dollar so wide receivers sometimes fill the extra positions, particularly in late-game situations in which the receiver's offensive skills can be put to good defensive use.
When the defense concedes short plays, an offense that can practice clock management effectively can score without executing the long pass the defense seeks to prevent. Some coaches avoid using the prevent defense and choose instead to continue playing the same defensive schemes that seemed to be working well to that point. John Madden once said, "All a prevent defense does is prevent you from winning."
By conceding to the offense many easy gains for short yardage but no big play, the prevent defense can make the end of the game uninteresting for fans.
The attempt to prevent a long-yardage play can be a victim of individual effort, as happened to the Denver Broncos in the 2012 AFC Divisional Round playoff game. With less than 40 seconds to play, the Baltimore Ravens needed a touchdown to tie the game and faced a third down from their own 30-yard line. Broncos safety Rahim Moore allowed Baltimore receiver Jacoby Jones to get behind him and catch a 70-yard touchdown pass from Joe Flacco. The Ravens went on to win the game in double overtime.