In American football, the pro set or split backs formation is a formation that was commonly used as a "base" set by professional and amateur teams. The "pro set" formation featured a backfield that deployed two running backs aligned side-by-side instead of one in front of the other as in traditional I-formation sets. It was an outgrowth of the original, three running back T-formation, with the third back (one of the halfbacks) in the T becoming a permanent flanker, now referred to as a wide receiver.
This formation was particularly popular because teams can both run and pass the football out of it with an equal amount of success. This is important because it keeps defenses guessing on what type of play the offense will run. Because the backs are opposite each other, it takes the defense longer to read the gap the offense will run the ball to.
Once the run has been established, it can be a very dangerous formation. Because of the real threat of a team running out of the pro-set, defenses must respect the play fake and play run. This pulls the safety to the line and opens up the middle of the field deep. Also, with both backs in position to "pick up" an outside blitz, the pro-set gives a quarterback an abundance of time to find an open receiver.
The set can be run with a single tight end and two receivers or no tight ends and three receivers.
A standard pro set places the backs about 5 yards behind the line of scrimmage, spaced evenly behind the guards or tackles. In this look teams may utilize two halfbacks, or one halfback and one fullback.
A variation of the pro set places the backs offset toward one side (either strong or weak). This look is almost universally used with one fullback one halfback. The backs line up closer to the line of scrimmage than in a standard pro set, about 3 yards deep. The fullback lines up directly behind the quarterback, in the spot he would normally line up in the I-Formation. The halfback then lines up behind either the left or right tackle.
The formation has lost its popularity at the college and professional level recently with the rise of shotgun split back formations. It remains common at the high school Junior Varsity and Varsity level.
At the NFL level, in the mid-to-late 2000s, the formation became used almost exclusively by West Coast Offense-based teams in occasional third down passing situations, and goal-line situations. In the early 2010s, the pro set almost completely disappeared from the NFL, however in the late 2010s it has been used once again as an occasional goal line and passing downs formation by West Coast Offense-based teams.