Defensive Line
The defensive line consists of one or two defensive tackles and two defensive ends who play outside the defensive tackles. The defensive line works with the linebackers to try to control the line of scrimmage. The 4-3 defensive formation, most commonly used in the NFL, employs two defensive tackles (and a defensive line of four men, with three linebackers behind them), while the 3-4 formation uses just a single defensive tackle, called the nose tackle (and a defensive line of three men, with four linebackers behind them). However, defensive ends in a typical 3-4 have responsibilities more similar to a 4-3 defensive tackle than 4-3 defensive ends.
On running plays, the goal is to tackle the ball carrier. The defensive line attempts to maintain their original formation (even spacing without holes), but also to prevent any members of the opposing offensive line from successfully engaging the linebackers, who chase down the ball carrier. The defensive tackles are usually the most skilled run defenders on the team.
On passing plays, the defensive line tries to reach the quarterback. Ideally, the defensive players are able to tackle the quarterback for a loss (a sack), but in practice the quarterback will usually manage to throw the ball before an actual tackle is made; the goal is thus to put pressure on the quarterback as quickly as possible to force him to throw the ball before he can find an open receiver. Defensive ends are usually the most skilled pass rushers on the team. In order to increase the pressure on the quarterback, teams will often have players other than the defensive line attempt to tackle the quarterback; this is called a blitz.
Because the defense does not know whether the offense is attempting to run a passing play or a running play (or whether a quarterback will give up on an attempt to pass and instead run with the ball), they must balance passing and running strategies: running around offensive linemen and avoiding contact may allow faster pressure on a quarterback, but it also leaves a hole in the defensive line and frees an offensive lineman to engage a linebacker, enabling a big running play.
Defensive linemen--particularly defensive ends--are called upon to do more running than offensive linemen, thus they usually tend to be somewhat smaller and faster.
Read more about this topic: Offensive Line Coach
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