The Dallas Morning News has a cute article, about how the first defensive call by Rob Ryan on the first defensive play of the first preseason game of Dallas in 2011 was the 43 Flex. I recall watching that play and thinking “psycho front”. And yes, Ryan has 4 players along the line of scrimmage and 3 players at linebacker depth, but what we’re going to do in this article is talk about about Tom Landry’s first two defenses, the 43 inside and 43 outside, and how they then morphed into the flex, to better use the talents of their All-Pro defensive tackle, Bob Lilly.

Dallas-Miami, SB VI, 4-3 inside line setup.

43 inside/outside. Inside, DTs rush "A" gap. Outside, "B" gap.

If you have the set “Vince Lombardi on Football“, then you have perhaps the best resource I can locate on the 4-3 inside and the 4-3 outside. Pages 174 through 185 cover these two defenses. The physical setup of the defensive line is the same in both cases. In the 4-3 inside, the defensive tackles rush into the “A” gaps and the middle linebacker is responsible for both “B” gaps. In the 4-3 outside, the defensive tackles rush into the “B” gaps and the middle linebacker is responsible for both “A” gaps. The front, from the offenses left to right, is a “5-2-2-5” alignment, with the tackles head up on the offensive guards, and the ends on the outside shoulders of the tackles. The middle linebacker is 1.5 yards deep, the strong side linebacker is nose onto the tight end if the tight end is separated, suggesting strong side sweep.

Vince Lombardi on the 4-3 inside

Vince Lombardi on the 4-3 outside

The ideas for the Flex came about after Bob Lilly’s move from left defensive end to right tackle. Dick Nolan describes it as one half of  the line playing a 43 inside, one half playing a 43 outside. To note, the  tackles in the inside/outside are flexed. In Tom Landry’s Flex, however, it depended on which side of the offense was “strong”, or likely to be  the side players would run to. Bob, in Peter Golenbock’s book, describes it as follows:

If I were on the weak side, I’d be head-up with the guard, right on the line of scrimmage, whereas the tackle on the other side would be three feet back. George Andrie would be right over the tackle and instead of being on his outside shoulder, he’d be head-up, three feet back. He would be keying my guard. I also keyed my guard.

Dallas flexed. DLT on the LOS because offense is strong left.

43 flex. Left to right, front is "4-2-2-5".

As Dick Nolan explains

Let’s say the other team tries the old Lombardi sweep. When that guard pulls and that center tries to choke back to get Lilly, he can’t get to him quick enough because Lilly can just go around him, and the center will fall down on his nose trying to block him. Lilly will be running right behind their guard, and Paul Hornung will be running the ball, and Paul Horning can’t come back, because  if he does, he’ll be running right back into Lilly…

To guard against the counter, the off side defensive end now plays a 4 technique as opposed to a 5. That end is responsible for the weak side gap that the off side defensive tackle has left behind.

When introduced, it caused a lot of confusion,  because Dallas soon came to  be able to play the  43 inside/outside from the Flex set. That was the upside, as no one knew what they were actually playing. The downside is the weak side defensive end’s pass rush was effectively stuffed whenever the Flex was played. By the late 1970s early 1980s, it became almost automatic for teams to pass when they saw the Flex. Consequently,  as Charlie Waters explains in Golenbock’s book, the Flex was played less and less.

In the 1990s, Dallas switched to the Miami 43, versions of which are still played today. A derivative of the Miami 43 is Ron Vanderlinden’s Stack defense, discussed briefly here.

And now we have Rob Ryan’s 43 Flex. No, it doesn’t look a bit like the Tom Landry defense, but does resemble, somewhat, the double eagle flex defenses that were popularized by Dick Tomey and Rich Ellerson. A screen shot and a diagram of Rob’s defense follows.

Rob's 43 flex, first play of preseason. Denver appears to have an 8 man line.

Rob Ryan's 43 Flex

Notes: updated due to typos, and a very nice article on Blogging the Boys that identified each player along this front. Further, the blog Compete in All Things has some Xs and Os on the modern 43 Flex.


It’s a short book, very much an outline as much as it is anything, and that is both this book’s blessing and curse. It’s a blessing because it’s packed with ideas, and it’s something of a curse in that the details of implementation are often left up to the reader. For the fan, it’s perhaps an easier read than Ron Vanderlinden’s tome, and so I think much more suitable for the curious but casual fan of defensive technique.

For those who aren’t familiar with the double eagle flex, it’s worthwhile noting  that this is an 8 in the box defense closely related to the 46 employed by the Ryan family. There are 5 men along the line, including one flexed 3 technique tackle. The strong safety, or rover, is a hybrid player, and I’ve depicted him below as a linebacker. But much like many run oriented modern defenses, this player has to both play linebacker technique and also defend the pass.

It wouldn’t be a recent review on “Code and Football” if we didn’t provide the reader with a diagram, so this is my representation of the Double Eagle versus the Ace formation, three wide.

Ace formation versus double eagle flex. High School field. DBs funnel receivers into FS.

Note that both the boundary corner and the rover are supposed to funnel their men into the free safety.

The book is 98 pages long, packed with shifts, mods, stunts, all described in that  brief synoptic style.