The 3-4 defense in football is a defensive alignment strategy consisting of three defensive linemen on the line of scrimmage and four linebackers in the defensive backfield. The 3-4 defense is used primarily as a means to protect against the run and pass, as it employs four linebackers who are watching for a pass or run play. The other four players on the field are defensive backs, usually two cornerbacks and two safeties in the deep defensive backfield.
The three defensive linemen line-up on the line of scrimmage opposite the offensive line while the four linebackers line-up three to four yards behind the defensive linemen. The defensive linemen and linebackers are all in an area called the box, an area about five yards wide and seven yards long extending into the defensive backfield. The offense may change the play they are going to run depending on how many defenders are in the box. The 3-4 defense is extremely versatile because of the ability to defend both the run and pass consistently, as well as blitzing the offense if need be.
The linemen are there doing what they always do, stuffing the line of scrimmage in case of a run and pressuring the quarterback in case of a pass. There are only three linemen in this defense, so their performance is vital to the success of the defense.
The linebackers usually have to be the most versatile player on the field, and this defense is no exception. They are the primary pass defenders over the middle and the pivotal defenders against runs that break through the line of scrimmage. Their play is probably the most important of all the defense.