Single set Back

Single set back (also known as the "Lone Setback" or "Singleback" or "Ace" formation or "Oneback" or "Solo") is an offensive base formation in American Football which requires only one running back (usually a halfback) lined up about five yards behind the quarterback. There are many on single back formations including two tight ends and two wide receivers, one tight end/three wide receivers, etc. The running back can line up directly behind the quarterback or offset either the weak side (away from the tight end) or the strong side (towards the tight end).

Washington Redskins coach Joe Gibbs first began using the single-back as a base offense in order to move defenders off the line of scrimmage. He found that having an extra receiver on the line forced defenses to choose their poison by either defending the pass or the run because they were not able to focus on both at once using the amount of personnel that they had on the field. He also then began utilizing sets of two tight ends as blockers mainly to protect his quarterbacks from Lawrence Taylor. He believed having an extra blocker on the line would make it easier to keep Taylor out of his backfield. By using different formations and motion before the ball was snapped, he also found that he could confuse defenders on whether the play would be a pass or a run.

This formation has gained popularity in the NFL as teams have started trading out a fullback, or blocking back, in favor of another wide receiver or tight end who is usually faster and better able to receive the ball, while still helping the run game with down-field blocks. The effectiveness of the formation is further increased if the team has athletic tight ends with good pass catching abilities, thereby increasing the versatility of the formation. It is, moreover, good for bootlegs and reverses. Many NFL teams will also run the same singleback formations out of the shotgun with the running back standing next to the quarterback.

Single-back offenses have gained popularity due to zone blocking and advanced defenses. There are several combinations of single back formations that are used in Division 1 and NFL football. Speed offenses will use single back because the defense still has to respect the run out of these formations since you can line up many tight ends and still have a down field passing game. Single back offenses create match-up problems in the defense. Linebackers will often have to cover receivers in passing routes while defensive safeties are used more to come up and stop the run on the line of scrimmage. Teams that run a single-back offense typically rely on quick receivers that run great routes, balanced tight ends (blocking/receiving), intelligent, shifty running backs, fast and intelligent offensive linemen, and a quarterback that can read defenses and make safe throws under pressure. Single-back offenses are more common in the NFL than in college or high school.

...
Wikipedia