The flexbone formation is an offensive formation in American football that includes a quarterback, five offensive linemen, three running backs, and varying numbers of tight ends and wide receivers. The flexbone formation is derived from the wishbone formation and features a quarterback under center with a fullback lined up directly behind the quarterback. There are two smaller running backs called slotbacks aligned behind the line of scrimmage on each side of the offensive line. The slotbacks are sometimes incorrectly referred to as wingbacks. But in order to be a wingback, there must be a guard, tackle and tight end all on one side of the center on the line of scrimmage and then the wingback off the line of scrimmage (as featured in the unbalanced formation diagram).