Football 3-4 Defense

What is the 3-4 Defense in Football?
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.
3-4 Defense Positioning
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.
3-4 Defense Roles and Responsibilities
In a 3-4 defense, each position group has separate responsibilities, but still aim to achieve the same goal: stopping the offense. Read on to learn more about the responsibilities of players in a 3-4 defense.
Linemen
The three linemen of a 3-4 defense do what linemen always do: stuff the line of scrimmage in case of a run and pressure 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. If the three linemen are able to pressure the quarterback successfully, there will not be much space for the quarterback to throw, due to the presence of four linebackers in the middle.
Linebackers
The linebackers on a defense usually have to be the most versatile players on the field, and the 3-4 scheme is no exception. The four linebackers in this scheme are the primary pass defenders over the middle and the pivotal defenders against runs that break through the line of scrimmage. Since there are more linebackers and fewer linemen in a 3-4, the linebackers will need to be able to pick up whether a play is a run or a pass much quicker.