Which Eagles QB is best at solving defenses?

The play has been delivered. The offense lines up. And the quarterback settles under center or in the shotgun.

What comes next, Eagles coach Chip Kelly said, is "huge."

The quarterback must make his pre-snap reads, and more often than not the most vital read will involve the safeties.

"It's huge understanding what the defense is doing, conceptually what we're trying to get accomplished," Kelly said last month. "There are certain run plays we can run versus any look, so the pre-snap read isn't that important.

"But there are certain things - some in the run game and a lot in the pass game - where you need to understand what you're going against."

Reading defenses is just one facet of playing arguably the most difficult position in sports. But its importance has grown, especially before the snap, as defenses have become more sophisticated and more adept at disguising their plans.

The Eagles quarterback competition will hinge on many factors, but the ability to dissect a defense's intentions will boost the candidacy of Michael Vick, Nick Foles, or Matt Barkley.

At Oregon, much of what Kelly ran on offense revolved around his quarterback's knack for making the correct pre-snap read in regard to the safeties. When he was asked at the NFL owners meetings in March whether he relied on advanced metrics in making evaluations, Kelly joked that he "was told there would be no math."

In truth, Kelly requires his quarterbacks to understand at least basic arithmetic. At a Nike coach of the year clinic in 2011, as explained in a Grantland article from last November, Kelly gave his general philosophy on pre-snap reads based on the safeties' positioning.

If a defense positions both its safeties deep to defend the pass, there should be only five defenders in the box if the offense has four receivers. More often than not, Kelly said, he would run the ball with five offensive linemen required to block only five defenders in the box.

If there is one safety high and six defenders in the box, the quarterback has to become involved in the play and "read" one of the defenders - by having the option to run and thus drawing the defender to him - essentially blocking the sixth man.

Seven men in the box and nary a safety deep forces a defense into man-to-man coverage, which Kelly believes he can exploit in one way with quick hitch passes.

It's unclear how much read-option Kelly will have in his pro offense. A lot will depend on the starter's skill set. But even without it, the premise is the same. Kelly isn't reinventing the light bulb here, but his quarterback has to be adept at reading defenses and sharp enough to audible at the line.

"There's a ton of math," Kelly said. "Do they have one more guy in the box? That's what defenses are trying to do. We've got to get one more guy in the box stopping the run game. If you have one more guy in the box, you have less guys deep.

"So that becomes a chess match within the game of how are they trying to defend us or are you going to try to play man coverage all day long and get an extra guy in there?"

There are always exceptions to his rule, and, of course, it remains to be seen how effective Kelly's approach will be at the next level. The NFL's best defensive coordinators have become experts at masking their objectives.

"When you think there's going to be someone fit in the front, and they're not fit in the front, they did a great job of disguising it," Kelly said. "That's part of your film study and your evaluation, making sure you put a plan together that can take advantage of that and you don't get fooled."

Vick has a longer NFL resumé than either Foles or Barkley, so, fair or not, it's easier to pick apart his past performances. Some have attributed his recent struggles to his reading of a defense, a criticism he bristled at in May.

"You don't last 12 years in the NFL not being able to read the defense," Vick said on WPEN-FM (97.5). "Those people who are talking and saying that are just ignorant, and they know nothing about football."

Last season, the Eagles took more off Vick's plate in terms of his pre-snap calling out of blocking assignments. That job fell to the center. But Vick's other pre-snap responsibilities, as they are for any quarterback, were the same.

"Playing quarterback in this league, there's a lot of pre-snap, post-snap on the quarterback," said Foles, who also had issues identifying defenses pre-snap last season as a rookie. "The defenses in this league are very good at hiding things, so you always have to be aware of the safety or whatever you are keying on with each play."

There is, of course, more to reading defenses than just locating the depth of the safeties. Quarterbacks have to account for many blitz packages, stunting and twisting linemen, and complex zones, among other things.

Kelly has been cautious when asked to assess how his quarterbacks with NFL experience performed before his arrival. He's watched the tape and seen all the bad (and good), but said he can't make an accurate evaluation yet.

"Unless you intimately understand exactly what they were told to do, then how can you make an evaluation whether that was the right thing to do or the wrong thing to do?" Kelly said. "It could have been, 'I don't care what happens. Throw the ball to the tight end.' And then you watch the film and go, 'Jeez, he forced that one in there.' Well, he may have been told to force that one in there."

Kelly will call the plays now. But that's where his involvement ends. It's up to his quarterback to make the correct pre-snap reads and execute his offense.

It could take up until the season opener - and perhaps longer - for Kelly to figure out who that quarterback will be.