Middle Linebacker
The middle or inside linebacker (MLB or ILB), sometimes called the "Mike" or "Mack",[14] is often referred to as the "quarterback of the defense".[15] Often it is the middle linebacker who receives the defensive play calls from the sideline and relays that play to the rest of the team, and in the NFL he is usually the defensive player with the electronic sideline communicator. A jack-of-all-trades, the middle linebacker can be asked to blitz (though they often blitz less than the outside linebacker), cover, spy the quarterback, or even have a deep middle-of-the-field responsibility in the Tampa 2 defense. In standard defenses, middle linebackers commonly lead the team in tackles. The terms middle and inside linebacker are often used interchangeably;[16] they are also used to distinguish between a single middle linebacker playing in a 4–3 defense, and two inside linebackers playing in a 3–4 defense.[17] In a 3–4 defense, the larger, more run-stopping-oriented linebacker is usually still called "Mike", while the smaller, more pass protection/route coverage-oriented player is called "Will". "Mikes" usually line up towards the strong side or on the side the offense is more likely to run on (based on personnel matchups) while "Wills" may line up on the other side or even a little farther back between the defensive line and the secondary.[citation needed]

Outside Linebacker
The outside linebacker (OLB), sometimes called the "Buck, Sam, and Rebel" is usually responsible for outside containment. This includes the strongside and weakside designations below. They are also responsible for blitzing the quarterback. Not only is the OLB responsible for outside containment and blitzing the QB, but they also have to perform pass coverage in the flats - sometimes called a drop. Outside linebackers pass coverages covers quick slants outside, in curls in the flats. The "flats" are the edge of the field closest to the sideline, from the line of scrimmage down about ten yards.

Strongside Linebacker
The strongside linebacker (SLB) is often nicknamed the "Sam" for purposes of calling a blitz. Since the strong side of the offensive team, is the side on which the tight end lines up, or whichever side contains the most personnel, the strongside linebacker usually lines up across from the tight end. Often the strongside linebacker will be called upon to tackle the running back on a play because the back will be following the tight end's block. He is most often the strongest linebacker; at the least he possesses the ability to withstand, shed, and fight off blocks from a tight end or fullback blocking the backside of a pass play. The linebacker should also have strong safety abilities in pass situation to cover the tight end in man on man situations. He should also have considerable quickness to read and get into coverage in zone situations. The strongside linebacker is also commonly known as the left outside linebacker (LOLB).

Weakside Linebacker
The weakside linebacker (WLB), or the "Will" in 4–3 Defense, sometimes called the backside linebacker, or "Buck", as well as other names like Jack or Bandit,[14] must be the fastest of the three, because he is often the one called into pass coverage. He is also usually chasing the play from the backside, so the ability to maneuver through traffic is a necessity for the Will. The Will usually aligns off the line of scrimmage at the same depth as Mike. Because of his position on the weakside, the Will does not often have to face large interior linemen one on one unless one is pulling. In coverage, the Will often covers the back that attacks his side of the field first in man coverage, while covering the weak flat in Texas Loop or hooks/curl areas in zone coverage. The weakside linebacker is also commonly known as the right outside linebacker (ROLB).