To the amateur eye, how different defenses are set up on the football field--and why they choose to align a certain way--can be confusing to grasp.
Coaches have different philosophies on defensive formations, based on the players they have, what they're expecting out of the opposing offense and what they feel most comfortable with.
In most cases, it's one of seven different formations. Here's a breakdown of each one and why coaches may choose them come game day, courtesy of the eteamz community:
The 4-3 is the most commonly used defense at the upper levels, including the NFL. At lower levels the 4-3 is not particularly popular because many coaches consider it weak against the run due to the fact there are only four down linemen. At the higher levels, the quality and size of the average down linemen makes this a non factor. In essence, if a team possesses the size, strength, and quickness necessary to run the 4-3 defense, it is a formidable defensive formation.
Besides the ever-present four down linemen (2 tackles and 2 ends), there are three linebackers--two to the inside and one at the outside shoulder of the tight end. Two cornerbacks and two safeties are the standard. Equally effective against most all offensive formations, the 4-3 is the default defense of choice for this author. It is easily modified for various offensive sets. The third linebacker (on the tight end) can cover the tight end, blitz, or cover any of the short zones to that side or the hook zone over the middle. The cornerbacks can blitz with the safety(s) assuming the corner's responsibilities. Or a corner can drop back in deep coverage allowing a safety blitz.
Because of its high flexibility, an offense will find it difficult to isolate a particular area or defensive player. If the 4-3 has a weakness, it is that the inside linebackers are the primary tacklers for runs between the tackles and they are of course four to five yards off the ball.
The 3-4 is designed to stop the short passing, ball control type offense. Naturally less than ideal against the run due to only three down linemen, this defense offers an extra defensive back for pass coverage.
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Consisting of a nose guard and two other down linemen, the coach has the task of deciding who the outside two linemen are--ends or tackles. Often one of the linebackers has zone pass coverage responsibilities, in effect employing five defensive backs. This is why the 3-4 is often referred to as the "nickel" defense. Having five defensive backs allows for random blitzing by one or more of these backs in order to maintain a sufficient pass rush. The 3-4 is susceptible to the inside run and is used primarily is situations where an interior run is not expected.
The 4-4 is designed to stop the wide running game as well as the short passing game.
The 4-4 uses four down linemen, four linebackers, two cornerbacks, and a safety. Stunts are a common component of this defensive set, usually with the some or all of the linemen stunting left or right and the inside linebackers stunting in the opposite direction. A wide range of possible stunts and blitzes are possible. The 4-4, also known as the "stack" defense, relies on quickness, particularly quickness in pursuit. In order to run the 4-4 on a regular basis, the interior down linemen must be players of considerable substance.