In American football, the pro set or splitback formation is a traditional formation, commonly a "base" set used by professional and amateur teams. In pro set formations, the running backs are lined up 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 is 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 is the most dangerous formation in football. 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.
A common variant of this formation removes the tight end and replaces it with a third receiver in the "slot" position. The formation is utilized to remove a defensive player from the tackle box to give the offense a 7-on-6 matchup.