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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. The alignment uses seven 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 will be uncovered downfield or can get behind them.
The prevent defense is virtually never used except when it seems clear that the offense must pass the football to gain long yardage quickly: on third-and-very-long, or with the offense trailing in the score and with time running out.
Types of prevent
The quarter defense has three down linemen, one linebacker, and seven defensive backs. The quarter defense gets its name as the next coin after nickel and dime in the sequence of defenses. Quarter or quarters can also be used to describe a type of zone pass coverage, in which four defensive backs divide the field into vertical quarters for zone coverage. 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
Half dollar defenses have eight defensive backs and are very rare and are often used to prevent a Hail Mary pass. Professional teams may not have enough defensive backs on the roster to play the quarter or half dollar, so wide receivers are sometimes used to fill the extra positions, particularly in late game situations where the receiver's offensive skills can be put to good defensive use.
In this case the defense will pull back into what is called a soft zone. They will most likely play in a nickel, dime, or quarter package. A soft zone means that all the safeties and cornerbacks are playing back, five or ten yards off the line of scrimmage. The free safety will often play as much as 20 yards back. There will be no jamming of receivers on the line. The zone means that each defensive back is responsible for an area of the field, so they're all watching the quarterback's eyes instead of running stride for stride with a receiver. It is very easy for the offense to make short plays against this defense, gaining four to eight yards per play, but it's almost impossible for the offense to make a big play of 20 or more yards against this sort of defense.
In the fourth quarter when there are only a few minutes left in the game and one team is winning by 7 or more points, it's common in the NFL for that team to go into a "prevent" defense. In a prevent defense the idea is to make the other team use up a lot of time. The clock is stopped when the player with the ball steps out of bounds, so the first goal of the defense is to keep everyone in bounds. The only danger to the defense is giving up a big play, for example a 25 yard or longer pass or run. It doesn't matter to the defense if the other team makes a lot of plays, and gains four to eight yards per play, as long as the clock keeps running.
The Denver Broncos famously botched the prevent defense in the 2012 AFC Divisional Round playoff game against the Baltimore Ravens. With less than 40 seconds to play, the Ravens needed a touchdown to tie the game and faced a 3rd down from their own 30 yard line. Broncos safety Rahim Moore allowed Baltimore receiver Jacoby Jones to get between Moore and the endzone, where Jones caught a 70-yard touchdown pass from Ravens quarterback Joe Flacco. The Baltimore Ravens went on to win the game in double overtime, and eventually the Super Bowl.
The prevent defense is rarely used on consecutive downs, or with a significant amount of time remaining. 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. John Madden once said, "All a prevent defense does is prevent you from winning."
The bend-but-not-break nature of the prevent defense tends to give the offensive team many easy gains but no big play, so the prevent defense can make the end of the game uninteresting for fans. Some coaches avoid using the prevent defense, choosing instead to continue playing the same defensive schemes that seemed to be working well to that point.