This article first appeared in Football Outsiders and is reprinted with their kind permission. It's an excellent site, I recommend you give it a look.
Offensive formations are rapidly evolving, with three-receiver sets, four-receiver sets, and multipurpose backs who can line up anywhere and catch the ball as well as they run it. But the way Pro Bowl rosters and media depth charts show each team's starting lineup hasn't changed in decades. Every team is still shown with two receivers, a fullback and a running back, the formation that became standard thanks to the development of the I-formation.
The I-formation has been the dominant offensive formation in the NFL and has stayed effective for decades because of the way it balances the running and passing attack. We'll get into just how that balance is achieved, but first a little bit of history.
To cap the 1940 season, Chicago Bears coach George Halas used the T-formation (with the fullback directly behind the quarterback and the two halfbacks on either side of the fullback) to beat the Redskins 73-0 in the NFL championship game. Before long other teams saw that the T was more versatile than its predecessor, the single wing, and they adopted the T for themselves. By 1952, the Steelers (the last holdouts) switched to the T, and the single wing was out of pro football.
But around the same time, Tom Nugent, the football coach at Virginia Military Institute, had a new idea. Instead of having halfbacks on each side of the fullback, he thought it made more sense to line up the halfbacks behind the fullback. With the offensive backfield in a straight line, they looked more like an 'I' than a 'T', and a new formation was named.
After five years at VMI, Nugent brought the I to Florida State in 1954, and it began to spread, most notably to John McKay, who used the formation to win the national championship at USC in 1962. Before long, NFL teams took notice, and the I became the most common set in pro football.
Coaches who frequently run the I-formation often extol its virtues with statements like, "We just want to line up and come right at 'em." But there's nothing inherently tougher about the I than there is about any other formation. What set the I apart from the T and many of the other formations of the early days of football is that it lent itself to a versatile offensive attack. That versatility is still valuable in the modern game.
A typical bread-and-butter play from the I-formation might have a name like "I right 32 iso". That means an I formation with the right side being the strong side - the tight end and the flanker (the wide receiver a yard behind the line of scrimmage) are lined up to the right, with a split end (the wide receiver on the line of scrimmage) on the left. The 32 means the halfback (or the 3-back) is getting the ball and running through the 2-hole (between the center and right guard). "Iso" is short for isolation, meaning the fullback is leading the running back through the hole and engaging the middle linebacker in an isolation, one-on-one block. (You know, the kind of block where the TV audience hears Ray Lewis complain that he was double-teamed.)
The offensive linemen block the players in front of them, with each tackle taking a defensive end, a guard blocking one defensive tackle alone and the other guard and the center double-teaming the other defensive tackle. The tight end blocks the strongside linebacker, and the two receivers might run a flag or a hook route to take the cornerbacks out of the play.
A play like I right 32 iso won't produce a lot of long runs, but that's not what it's designed for. If the right personnel executes it properly, it will accomplish one of two things: Either it will consistently allow the offense to gain four yards a pop (and an offense that can do that will march right down the field) or it will force the defense to bring the strong safety close to the line of scrimmage for run support. When TV announcers describe this, they call it "eight in the box, " and it means the defense is susceptible to the other bread-and-butter play of the I-formation: the play-action pass.
When the offense sees the strong safety playing run, it becomes clear that it's time to run the play action. The linemen and fullback have the same blocking reponsibilities and try to make it look like a running play. The quarterback fakes a handoff to the running back, but instead of giving up the ball he goes into his seven-step drop. With the strong safety playing run support, the cornerback has no help in coverage on the flanker, who becomes the primary receiver on the play.
Let's talk a little more about the fullback, who usually lines up about four yards behind the quarterback, with the running back usually about three yards further behind. At seven yards behind the line of scrimmage, the running back can easily survey the field to see where the defensive players are lining up, and he can get a full head of steam so that he's at full speed when he hits the line of scrimmage. The running back also has time to watch blocks develop and can find cutback lanes.
The flipside, of course, is that a handoff to the running back in the I takes longer to develop than a handoff to the fullback, and that gives the defense more time to react. That's why in the early days of the I- formation, the fullback regularly carried the ball, especially in short-yardage situations. Today, however, football is a much more specialized game, and I-formation fullbacks are blockers almost exclusively and rarely carry the ball.