Dime coverage (or the dime formation) is a defensive alignment consisting of six defensive backs. This defense is used mainly on passing downs, especially when the offense has four wide receivers on the field.
The dime formation usually features three down linemen, two linebackers and six defensive backs (3-2-6), or four down linemen, one linebacker and six defensive backs (4-1-6). The extra defensive backs replace linebackers on the field.
Sixth Defensive Back
The sixth defensive back added during dime coverage is known as the dime back. In the nickel formation, there are five defensive backs, with the fifth being known as a nickel back (plus two cornerbacks and two safeties). Since the fifth and sixth defensive backs in the dime are essentially nickel backs, and two nickels make a dime, this explains how the formation got its name.
When the opposing football team is far behind and must pass to narrow the score, the defense will often counter by using dime coverage. While the dime can be vulnerable to a rushing attack (given the lack of linebackers), defenses protecting a lead are usually willing to give up yards on the ground in exchange for preventing the big play.
Dime coverage is also sometimes referred to as the dime package, and simply means it’s a particular defensive unit with its own unique players, set off from the base defense.
For a more thorough explanation of NFL defenses and formations, please read our entry entitled "Football Defense", which is located in the table to your right, third link from the top.