When a quarterback prepares for a football game, he wants to be able to read the defense — look at a specific defensive alignment and instantly know which offense will or won’t succeed against it.
Many quarterbacks are taught to read the free safety, or the safety positioned deepest in the secondary, the part of the field behind the linebackers that the safeties and cornerbacks are responsible for. If the free safety lines up 5 to 7 yards deeper than the other defensive backs, the safeties are probably playing a zone defense.
The quarterback can also figure out the defense by looking at the cornerbacks:
A quarterback knows that the defense is playing zone if the cornerbacks are aligned 10 yards off the line of scrimmage.
If the cornerbacks are on the line of scrimmage, eyeballing the receivers, they’re most likely playing man-to-man.
Knowing whether the defense is playing zone or man-to-man is important to the quarterback because he wants to know whether he’s attacking a zone defense or man-to-man alignment with his pass play.
Although a defense may employ 20 to 30 different pass coverages in the secondary, four basic coverages exist. Because most defenses begin with four players in the secondary, the coverages are called cover one, cover two, cover three, and cover four:
Cover one: One deep safety is about 12 to 14 yards deep in the middle of the field, the two cornerbacks are in press coverage (on the line of scrimmage opposite the two receivers), and the strong safety is about 5 yards deep over the tight end. Cover one is usually man-to-man coverage. A running play works best against this type of coverage.
Cover two: Both safeties are deep, 12 to 14 yards off the line of scrimmage. The two cornerbacks remain in press coverage, while the two safeties prepare to help the corners on passing plays and come forward on running plays. A deep comeback pass, a crossing route, or a swing pass works well against this type of coverage.
Cover three: Has three defensive backs deep. The free safety remains 12 to 14 yards off the line of scrimmage, and the two cornerbacks move 10 to 12 yards off the line of scrimmage. Cover three is obvious zone coverage. The strong safety is 5 yards off the line of scrimmage, over the tight end.
This cover is a stout defense versus the run, but it’s soft against a good passing team. In this cover, a quarterback can throw underneath passes (short passes to beat the linebackers who are positioned underneath the defensive back’s coverage). Staying with faster receivers in this area is difficult for some linebackers to do.
Cover four: All four defensive backs are off the line of scrimmage, aligned 10 to 12 yards deep. Some teams call this coverage “Four Across” because the defensive backs are aligned all across the field.
The cover four is a good pass defense because the secondary players are told to never allow a receiver to get behind them. If offensive teams can block the front seven (a combination of defensive linemen and linebackers that amounts to seven players), a running play works against this coverage.