A much-heard complaint about the Eagles in general and Billy Davis in particular is that he has changed over from the 4-3 that we all knew and loved to a 3-4, and worse still, to a two-gap 3-4 at that. The personnel doesn't fit, goes the argument. People are playing out of position; people don't understand their roles; the pass rush is non-existent, and so on. Worst defense in the league, and you can look it up.

Of late, though, there's been a glimmer. This is actually a strong run defense now, as the D linemen learn their roles and responsibilities. The two best players on the defense have been Thornton and Cox and they are perfect for the 3-4 DE position. Perfect. They just had to learn on the job and that has not always been perfect. As Davis has said, they're learning to play their technique.

So, let's start at the beginning. Is it one-gap or two-gap?

Clearly two-gap. The NT and both DEs are aligned head up on the center and offensive tackles. Classic stuff. Bill Parcells would love it. Ryans has moved up over the left guard while Kendricks is squared on RGIII and seven yards deep. Ryans is threatening to blitz. Note the spacing of the defensive linemen, both horizontally and vertically.

Ryans does come on the blitz but, this is a run blitz. He's not trying to hit a seam; he's attacking the guard. In fact, all the linemen are attacking blockers. Note their spacing. They are not bunched up, their shoulders are square, and there are no running lanes. They are all on the same horizontal plane. There is no cutback lane and none of the offensive linemen are leaking through to block Kendricks. Baltimore did this for years with Ray Lewis. The "other" ILB would act much like a fullback and attack the backside guard, who was usually the lineman whose job it was to block Lewis. Ryans is better equipped to fill this role than Kendricks, simply because he is bigger.

The defensive line has maintained their spacing; there are still no cutback lanes, and the RB is running out of real estate. The offensive linemen are losing contact with their blocks and the defenders are still squared to the line of scrimmage.

Barwin is actually the first defender to get to the back but Kendricks is in perfect position, The linemen kept him clean and the runner had nowhere to go.

Here's another: One-gap or two gap?

Dallas has a three TE one WR set, a strong running formation. The Eagles play with their base defense. This time, the NT is slightly shaded to the strong side of the formation but still basically playing two-gap, but what's this? Both DEs are in the B gaps, so they're playing one-gap. Both DEs are inviting, almost demanding a double team. With smaller DEs, Dallas might assign just one blocker to the end, but these are big mobile guys.

Dallas does double team both ends. Again, note the horizontal spacing. With no linemen reaching to the second level, both ILBs are clean.

The middle is now officially a train wreck. Thornton has driven his double team backwards. Cox has three guys piled up, and note that the D linemen have maintained their horizontal spacing. The ILBs are still clean.

This is actually a nice piece of running. The RB is being patient, waiting for something to open up.

Ah! A cutback lane. One of his linemen has leaked through to block Kendricks. Alas! He's got Cole right on him and Sopoaga, the only D lineman who two-gapped on this play has closed the cutback lane.

Play over. This is a nice illustration of one of Chip Kelly's principles: he wants versatile players. Here the base defense has stopped the Dallas power package, using three defensive linemen.

Here's another. One-gap or two-gap?

The NT and LDE are clearly playing two-gap, but Cox is in a 3-technique, occupying two blockers to the strong side of the formation.

The Giants double-team both DEs. At this point both ILBs are clean, Beatty is trying to come off Cox to block Cole, who has beaten the TE with an inside move. Unfortunately, Boothe is trying to come off Cox to block Ryans. The horizontal spacing by the D linemen remains intact.

Jacobs has no chance as Cox meets him in the hole. If Boothe hadn't come off Cox to block Ryans, it would have been Ryans in the hole. If Beatty hadn't come off Cox, it would have been Cole.

What happens if you don't maintain your horizontal spacing?

The Giants are in a strong right with a TE on the wing. Thornton and Sopoaga are playing two-gap, covering the TE and RG, but Cox is lining up in the weakside A gap.

At the snap, Cox attacks the center. The spacing of the linemen is all wrong now and there is a huge hole on the defensive right side. The ILBs won't be clean for long.

Cox runs himself right into the block, making it easy for Boothe to come off him and go for Ryans. Meanwhile, Kendricks has Beatty in his face with no protection. If Cox had maintained his spacing, one or both ILBs would be clean.

Cox has taken himself out of the play and Jacobs cuts back into the vacated area. Both ILBs have linemen in their faces.

Jacobs gains seven yards on first down, essentially because the D linemen did not maintain their spacing. That's big yardage inside the red zone.

In a 4-3, one or more linemen are assigned to penetrate. To a certain extent, they free-lance. Not so in the 3-4. Gaps and spacing are the keys to the 3-4.

by David (JR)