12 personnel means that there is one running back, two tight ends, and two receivers on the field. Having two tight ends gives the offense flexibility in both the run and pass game. Defenses are often forced to match the bigger offensive personnel with larger linebackers. Moving two tight ends around the formation can also help set the strength of the defense and overload one side of the field — especially with quick shifts or motions.
For a quick overview on personnel naming, click here.
12 personnel can put a lot of stress on the defense with the threat of running the ball versus attacking vertically. With two tight ends, the offense now has eight total gaps they could run to. In turn, that motivates defenses to bring more defenders in to defend those gaps and leaves space vertically and outside.
The strength of 12 personnel is in its ability to help make the defense predictiable. You’ll often see Cover 3 with an extra safety down at the line of scrimmage to help against the run. You’ll also be able to create a predictable picture in the run game against even fronts. If you know how the defense is going to align, that makes the job of the offense that much easier.