Tighten those splits

The most common way to describe a basic defensive formation is by stating the number of linemen involved followed by the number of linebackers. The number of defensive backs is usually not mentioned, though if it is, (such as in the "3-3-5"), the number typically appears after the number of linebackers, thus the formula would go (# of linemen)-(# of linebackers)-(# of defensive backs (if stated)) in these situations. This naming rule does not always apply when the personnel for a certain formation is lined up in a way that changes the function of the players in the defense. A good example to help explain this would be the "3-5-3," which actually uses the 3-3-5 personnel, but has the five defensive backs arranged with "3 deep", thus grouping the other two defensive backs with the linebacker group.

By far the most common alignments are four down linemen and three linebackers (a "4-3" defense), and three down linemen and four linebackers ("3-4"), but other formations such as five linemen and two linebackers ("5-2"), and three linemen, three linebackers, and five defensive backs ("3-3-5") are also used by a number of teams.

On plays where the defense expects the offense to pass, naming emphasis is often placed on the number of defensive backs. In a basic 4-3 or 3-4 defense, there are four defensive backs on the field (2 cornerbacks (CB), 1 strong safety (SS), and 1 free safety (FS)). When one of the linemen or linebackers is removed and an additional defensive back is added, common alignments of these five defensive back packages are the "nickel" package, which includes 3 CB, 1 SS, and 1 FS, and the "3-3-5," which is a nickel package variant that includes either 2 CB, 2 SS, and 1 FS, or 3 CB, 1 SS, and 1 FS like the standard nickel package. When a sixth defensive back is inserted, it is known as a "dime" package (4CB, 1SS, 1FS). In rare instances when a seventh defensive back is inserted, it is known as a "quarter" package (5CB, 1SS, 1FS or 4CB, 2SS, 1FS).[5]

As with offensive formations, there are many combinations that can be used to set up a defense. Unusual defensive alignments are constantly used in an effort to neutralize a given offense's strengths. In winning Super Bowl XXV, the New York Giants played with two down linemen, four linebackers and five defensive backs, a strategy that prevented their opponents, the Buffalo Bills, a team with a strong passing game, from completing long passes. In a 2004 game, the New England Patriots used no down linemen and seven linebackers for two plays against the Miami Dolphins.

Some of the more familiar defensive formations include:

4-3
3-4
5-2
4-4
3-3-5
5-2-4
Nickel
Dime
Quarter or Prevent
"Eight in the box"