Linebackers are categorised as Mike, Will, or Sam linebackers, which are abbreviations for middle, weak-side, and strong-side linebackers, respectively. Both 3-4 and 4-3 systems use weak-side and strong-side linebackers, with the strong side corresponding to the quarterback's dominant hand. In a 3-4 defense, the weak side is called the "linebacker" because it lines up directly across from the tight end. The term "middle linebacker" is used in 4-3 defenses when referring to the position behind the two outside linebackers. There are three ways that a linebacker can be classified: by position, by role, or based on structure of the organization assigning them. A linebacker can be assigned a position based on where they line up on the field. There are also certain positions that typically require a certain type of linebacker. For example, inside linebackers usually have to be strong players who can run downfield and make tackles in the box (i.e., in the defensive backfield). Outside linebackers usually have to be bigger players who can cover wide receivers and running backs. Linebackers can also be categorized by role rather than position. Some coaches will refer to a player who primarily makes the sideline to sidelines runs as a "weak-side linebacker", while others may call him either the "Mike" or the "Sam" linebacker depending on what system they are using. Finally, some organizations have formed special teams units that include linebackers.