Imagine a castle which is being attacked by an invading army. This castle has three fortifications: a thick stone wall which protects the castle living quarters and treasury, a moat which surrounds those walls, and an earthen mound which encircles the moat. Similarly in football, there are three layers of defensive protection to prevent the other team's offense (the invaders) from putting the ball in your end zone (the castle).

The three defensive layers are the linemen, the linebackers, and the secondary. Defensive formations vary based on how many players make up each layer. Formations take their names from the number of down linemen and linebackers. These are the basic football defenses:

The 3-4 defense uses three down linemen (layer one) and four linebackers (LBs)(layer two). This leaves four defensive backs in the secondary (layer three). The line is made up of two defensive ends (DE) flanking a defensive tackle (DT), known as a nose tackle (NT) or nose guard. The contingent of linebackers is made up of two inside LBs who line up behind the middle of the line, and two outside LBs (OLBs) who typically line up behind and to the side of the DEs. This defensive formation is traditionally topped off with two corner backs (CBs), a strong safety (SS), and a free safety (FS).  The 3-4 is dependent upon the speed and playmaking abilities of its LBs.

The 4-3 swaps a linebacker for an additional down lineman, in this case another defensive tackle.  There is a middle linebacker (MLB) and two OLBs who back the four-man line.  These OLBs are commonly designated “strong” side (covering the opposing tight end) and “weak” side (on the side without the tight end).  Like the 3-4, the secondary also features two CBs and two safeties.

The nickel formation utilizes a four-man line like the 4-3, but has only two LBs. There are five defensive backs hence the name “nickel”.  A variation on the nickel is a 3-3-5 package, adding a LB instead of a fourth lineman. The nickel formation is used in an obvious passing situation.

The dime trades out a linebacker for yet another DB to match up against four or five wide receivers in a 4-1-6 alignment.  Sometimes defensive coaches can use a three-man line for a 3-2-6 or 3-1-7.  These alignments sacrifice a strong defensive pass rush in favor of more coverage of receivers.

Choosing the right defensive formation is a critical part of a winning game strategy.