The defense is the eleven players whose job is to stop the offense from scoring and win the ball back. It works in three layers: a line that attacks at the front, linebackers behind them, and defensive backs guarding the deep field. When the other team has possession, this is the unit on the grass, and everything it does is aimed at ending their drive.
What the defense is trying to do
The defense has the opposite job to the offense: stop the ball moving forward, and take it away if it can.
It does that in a few ways. It tackles the ball-carrier before he gains ground. It breaks up passes before a receiver can catch them. It pressures the quarterback, and best of all sacks him, tackling him before he can throw. And when it can, it forces a turnover, either catching a pass meant for the other side (an interception) or knocking the ball loose (a fumble), handing possession straight back to its own offense.
The three levels of a defense
A defense is built in three layers, front to back.
The defensive line is the front: three or four big players who fire off at the snap to stop runs at the source and chase the quarterback. Behind them are the linebackers, the flexible middle of the defense, who stop runners breaking through, drop back to cover short passes, and often blitz. The middle linebacker is frequently the one running the defense, wearing the green dot that receives the coach’s signal. At the back is the secondary, the defensive backs: cornerbacks who track the receivers and safeties who patrol deep as the last line before the end zone.
The defense that lies
Here is the part that makes defense an art rather than a wall. The offense knows its own play; the defense does not. So the defense disguises.
Before the snap it shows the quarterback one picture, then does another after it. It can crowd the line as though a blitz is coming, then drop back into coverage. It can look as if every defender is guarding a man, then shift into guarding zones. The quarterback has only seconds to read what is real and what is a bluff, and a good defense makes sure he reads it wrong.
Man coverage and zone coverage
When defending the pass, a defense picks one of two basic ideas.
In man coverage, each defender is assigned a specific receiver and follows him wherever he goes. It is tight and simple, but if a defender loses his man, there is nobody behind him. In zone coverage, each defender guards an area of the field instead of a person, watching the quarterback and reacting to whoever enters his patch. Zone keeps everything in front and invites shorter throws; man squeezes the receivers but gambles on one against one.
What a blitz is
Normally a defense rushes the quarterback with its linemen and drops everyone else into coverage. A blitz sends extra rushers on top of the line, usually a linebacker or a defensive back, to reach the quarterback faster.
It is a gamble. Send more and you are likelier to hit the quarterback before he throws, but you leave fewer defenders in coverage and a receiver may come open behind it. A well-timed blitz wrecks a play; a badly read one gives up a big one.
Base fronts: the 4-3 and the 3-4
You will hear defenses described by two numbers, like “4-3” or “3-4”. The first is the number of linemen, the second the number of linebackers. A 4-3 leans on its front four to pressure the quarterback; a 3-4 uses an extra linebacker for flexibility and disguise. Against pass-heavy offenses, a defense also swaps a linebacker for an extra defensive back, called a nickel, or two of them, called a dime.
What the numbers mean
Sacks, interceptions, tackles, passes defended: the defense fills its own column of the box score. What those figures are really telling you is decoded in NFL defensive stats. And the unit all of this is built to stop is how the offense works, the other half of how football is played.
Frequently asked questions
What does the defense do in football? The defense is the unit without the ball, and its job is to stop the offense from scoring and win possession back. It does this by tackling the ball-carrier, breaking up passes, pressuring and sacking the quarterback, and forcing turnovers such as interceptions and fumbles. It lines up in three layers: the line, the linebackers and the defensive backs.
What is a blitz in football? A blitz is when the defense sends extra rushers at the quarterback, on top of the usual linemen, most often a linebacker or defensive back. The aim is to reach the quarterback faster and stop the play before he can throw. The risk is that it leaves fewer defenders in coverage, so a receiver may come open if the quarterback gets the ball away.
What is the difference between man and zone coverage? In man coverage, each defender covers a specific receiver and follows him around the field. In zone coverage, each defender guards an area instead, reacting to whoever enters it. Man coverage is tighter but riskier if a defender is beaten; zone coverage keeps the play in front but gives up shorter passes.