Running back
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A running back (RB) is the position of a player on an American or Canadian football team who usually lines up in the offensive backfield. The primary roles of a running back are to receive handoffs from the quarterback for a rushing play, to catch passes from out of the backfield, and to block.
There are usually one or two running backs on the field for a given play, depending on the offensive formation. A running back may be a halfback (HB for short, in certain contexts also referred to as a tailback) or a fullback (FB).
Halfback/tailback
The halfback or tailback position is one of the more glamorous positions on the field, as it is often integral in both the passing and running attack. A well-rounded halfback is commonly viewed as a requirement for a team's success. He is responsible for carrying the ball on the majority of running plays, and may frequently be used as a receiver on short passing plays. In today's game, an effective halfback must have a superior blend of both quickness and agility as a runner, as well as sure hands and shrewd vision upfield as a receiver. More and more quarterbacks depend on halfbacks as a dump-off receiver when primary targets downfield are covered. Occasionally, they line up as additional wide receivers. When not serving either of these functions, the primary responsibility of a halfback is to aid the offensive linemen in blocking, either to protect the quarterback or another player carrying the football. On some rare occasions, running backs are used to pass the ball on a halfback option play or halfback pass. No position in American football can perform his duties successfully without the help of other players. Like the wide receiver, who generally cannot make big plays without the quarterback passing to him (with the exception of the end-around or a reverse), the running back nearly always needs good blocking from the offensive line to successfully gain yardage. Also, a running back will generally have more rushing attempts than a receiver will have receptions, which is mainly because a receiver must catch a pass to obtain a reception, while a running back usually takes a direct handoff or short pitch from the quarterback before registering a rushing attempt.
Note that the difference between halfback and tailback is simply the position of the player in the team's offensive formation. The halfback lines up approximately half-way between the line of scrimmage and the fullback (similarly, quarterbacks line up a quarter of the distance between the line of scrimmage and the fullback). Because the halfback is usually the team's main ball carrier (while the fullback is primarily a blocker), many coaches see fit to position the halfback behind the fullback (at the "tail end" of the formation). In this case, the halfback becomes a tailback.
Fullback
In most college and professional football schemes, fullbacks carry the ball infrequently. Currently, they are primarily used to aid the offensive linemen in blocking. On most running plays, the fullback leads the halfback, attempting to block potential tacklers before they reach the ball carrier. Fullbacks are sometimes used in passing plays, although they typically protect the quarterback. Fullbacks are technically running backs, but today the term "running back" is usually used in referring to the halfback or tailback. Although fullbacks currently are rarely used as ball carriers, there was once a time when they ran the ball as frequently as running backs.
In high school football, where the offenses are simpler and player sizes vary greatly, fullbacks are still frequently used as ball carriers. In many high school and in some college football scheme known as the "Triple Option," the fullback is considered a primary ballcarrier for this offense. The fullback in this particular offense plays a unique role by establishing an inside running threat during a triple option. Triple Option scheme fullbacks require excellent ballcarrying skills and employ a tenacious running style, but often lack the receiving skills of their counterparts in other schemes. College teams such as Navy and Air Force have employed the triple option scheme with success.
While in years past the fullback lined up on the field for almost every offensive play, teams often opt to replace the fullback with an additional wide receiver or a tight end in modern football. Fullbacks in the National Football League rarely get to carry or catch the ball since they are used almost exclusively as blockers. Their talent and value to a team is usually judged by the success of the team's halfback or by how many times the team's quarterback is sacked, similar to the way offensive linemen are judged. Fullbacks are also still used occasionally as rushers on plays when a short gain is needed for a first-down or touchdown, as they are large and powerful and therefore effective at breaking through the line for a short distance. Jim Brown, Marion Motley, Franco Harris, and Larry Csonka were great fullbacks and some of the pioneers.