The Swiss army knife is one of the most versatile tools ever invented, and Kelly is a big, big fan of versatility.
He loves players who are human Swiss army knives; who, to borrow a popular Kellyism, have more than one tool in their tool box.
Wide receivers and tight ends who can line up inside or outside, and who can contribute as blockers in the running game as well as targets in the passing game.
Running backs who can line up anywhere in the formation and create mismatches against linebackers and safeties.
Safeties who can cover slot receivers. Cornerbacks who can help out in run support.
Defensive linemen who can play inside or outside. Outside linebackers who can rush the passer and set the edge against the run. Defensive line coaches who can double as Santa Claus at the team Christmas party.
Versatility is the foundation of Kelly's tempo offense. He can prevent opposing defenses from substituting, while his offense is able to run out of several different formations with the same personnel group.
"A big part of offensive football today is matchups," said center Jason Kelce, whose team will play its first preseason game tonight against the Bears in Chicago. "If you can get a good one-on-one matchup or a good matchup with a receiver on a safety or a tight end on a linebacker, we're going to take advantage of that.
"That's one of the good things about forcing a defense to put personnel out on the field and then hitting a [fast] tempo with them and making it really hard for them to sub into personnel packages they like to get into.
"Tempo not only makes the defense more tired, but makes it a lot harder for defensive coaches to get into those specialized blitz packages and those things they like to do to try and confuse offensive lines and get sacks on the quarterback."
Once upon a time, when an offense put two tight ends on the field, the defense usually could safely assume that it was going to be a run play. But the Eagles have three excellent receiving tight ends in Brent Celek, Zach Ertz and James Casey.
Last year, the Eagles used two or three tight-end sets 288 times. One hundred seventy-seven of those plays were run plays and 111 were pass plays.
If the defense stays in its base alignment to stop the run, they have to defend Celek and/or Ertz with a safety or linebacker. If they play nickel, the Eagles can hand the ball off to LeSean McCoy or Darren Sproles.
"It allows us to play as fast as we want to play," Ertz said. "I don't have to be in a specific spot, per se. I can be all the way out wide. I can be in the slot, or [attached to the tackle] at tight end.
"We've got three tight ends who can do that. Then you've got Darren [Sproles], who adds another dimension to the offense."
Sproles' addition to the offense is going to create a whole new set of complications for defenses, who constantly will have to worry about how they're going to cover him as a receiver, particularly if they aren't able to make personnel changes because the Eagles are running tempo.
And the Eagles will be running tempo. They finished just 13th in the league in offensive plays per game (64.8) last season. But that was because: a) their defense had a lot of trouble getting off the field on third down; and b) the Eagles tended to score quickly and didn't have a lot of long drives. When they had the ball, though, they didn't loiter. They averaged a play every 24.0 seconds, which was the best in the league.
Kelly has made it clear that, while he likes to play fast, he doesn't pay any attention to how many plays per game the Eagles are averaging.
"I don't think plays-run and playing fast are correlations," he said. "I've never been a plays-run guy. I think that's an overblown thing. I've always kind of chuckled at this whole deal of, he wants to run a ton of plays. I've never wanted to run a ton of plays. All I've ever wanted to do is score a ton of points."
Last year, the Eagles ran more plays than their opponent in just five of their 16 regular-season games. In their 10 wins, they ran more plays five times, and fewer or as many plays five times. In their six losses, they ran fewer plays than their opponent five times and the same amount once.
Against the Raiders in Week 9, the Eagles ran 35 fewer plays than Oakland, yet beat the Raiders by 29 points.
They ran a season-high 53 plays in the first half of their 33-27 Week 1 win over the Redskins, but ran 40 or more plays in a half just four more times the rest of the season.
"I don't care if we run 50 plays a game or 100 plays a game," Kelly said in June. "There are going to be different factors that go into it. Part of winning the game is managing the lead when you're in the second half of a game when you have a three-touchdown lead. In that situation, you don't want to go out there and run plays 15 seconds at a clip."
All that said, it's pretty safe to assume that the Eagles will average more than 64.8 plays per game in their second year in Kelly's system.
"There are certain situations where we really want to play faster," he said. "As a group, with a better understanding of our offense in year 2, I think we can improve offensively. We do have the ability to play faster. And there are going to be certain times when you want to be able to do that."
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