Who are the best play-callers in the NFL? It’s not a hard question to answer. Everyone pretty much agrees that Sean McVay, Josh McDaniels, Kyle Shanahan, Andy Reid and Sean Payton make up the top-five in some order.

Here’s a question that is far more difficult to answer: What makes those guys so good? Sure, their teams score a lot of points, but how do they do it?

Every coach in the NFL is pretty much running the same plays, so what separates the geniuses from the rest?

The simple answer: Observation.

The key to good play-calling is creating mismatches. That’s why coaches spend dozens of hours watching film every week: They want to know what they have to do to get the defense to play a certain coverage in a certain situation and how to get a certain defender matched up on one of their playmakers.

Play-calling is often described as an art, but it’s more of a science. At least for the best coaches in the league. At its core, a play-call is really just testing out a hypothesis based on observations made during the week when studying film and throughout the game on Sunday. Those coaches we listed above just do a better job of making those observations and coming up with sound hypotheses.

Let’s take a look at a few examples that perfectly illustrate the science of calling plays…

The first example comes from the Rams’ win over the Vikings on Thursday night. Los Angeles scored five touchdowns in the game, but the most important play of the night may have been this innocuous third-down conversion on the Rams’ opening drive.

It’s a nice little gain to keep the drive going, but nothing more, right?

Well, that’s not what McVay sees. He is looking to see how the Vikings are going to play a 3-by-1 formation — that’s three receivers to one side and one receiver to the other. Before the snap, the Vikings show a defense that suggests they are in “quarters” coverage, with four defensive backs manning the deep zones and the three defenders (in this case, two linebackers and the nickel corner) patrolling underneath.

After the snap, McVay sees how the Vikings will adjust their coverage to the 3-by-1 formation. To avoid mismatches on the overloaded side, safety Harrison Smith (No. 22, toward the bottom of the picture) is responsible for ranging over to cover the No. 3 receiver if he goes vertical. That makes the corner, Xavier Rhodes, solely responsible for the single receiver to the left side, and the linebacker, Anthony Barr, responsible for the back, Todd Gurley …

Here’s a diagram of the coverage the Vikings are playing, via Match Quarters:

Now McVay knows what the Vikings will do if he throws out that same personnel grouping and formation later in the game. He doesn’t wait long to use that information to produce the perfect play call.

Fast-forward to the end of that opening drive. The Rams are now on the Vikings’ 8-yard-line. McVay has come up with his hypothesis:

If the Rams come out in a 3-by-1 formation, the Vikings will play that same coverage.

If the No. 3 receiver runs a vertical route, he’ll draw Smith out of his position.

Now if the backside receiver runs a clear-out route (luring the corner toward the sideline), that will leave Gurley one-on-one with Barr in space — a mismatch the All-Pro running back should win easily.

McVay’s tested out his hypothesis and his confirmation came in the form of an easy touchdown pass…

McVay wasn’t done exploiting that coverage. In the third quarter, he called for another 3-by-1 formation, and, sure enough, the Vikings played the same coverage. Smith (the safety toward the bottom of the frame here) again focuses in on the No. 3 receiver, leaving Brandin Cooks one-on-one with Trae Waynes, who gets burned for a long touchdown…

Science!

Example No. 2 comes from the Patriots-Lions game in Week 3. Matt Patricia got the better of Josh McDaniels overall — mostly by baiting New England into running the ball with his use of personnel, a la Bill Belichick in Super Bowl XXV — but McDaniels still held his own in the matchup.

Whenever the Patriots had two tight ends on the field, the Lions would match them with their nickel package (five defensive backs). Teams normally would play base defense (four defensive backs) in that situation, but Patricia wanted New England to run the ball so he played with lighter personnel to entice them to do so.

Even with the size advantage, the Patriots still had trouble running the ball inside. They just couldn’t block the Lions defensive tackles or Christian Jones (No. 52), who is the team’s best linebacker against the run.

The team’s other linebacker in nickel, Jarrad Davis, is the better athlete but can’t get off blocks, which is why Detroit lines him up on the weakside and Jones on the strong side.

But when the Patriots motioned a tight end from one side to the other, Davis and Jones didn’t switch places. McDaniels realized he can use motion to dictate which linebacker would play on the strong side.

So here’s the hypothesis McDaniels came up with based on that observation:

If he uses a two-tight end set, the Lions will play nickel and set Jones to the strong side.

If he motions Rob Gronkowski across the formation, Davis will become the strong side linebacker.

After the motion, the Patriots will have two good blocking tight ends (Gronk and Dwayne Allen) and an offensive lineman to block the defensive end, Davis and a cornerback to the left. That’s a mismatch in New England’s favor.

By calling a perimeter run, the Pats take the defensive tackles out of the equation and force Jones to range over the top to get in on the play.

Here’s how it played out on the field…

That’s a lot of thought and effort to produce a 12-yard gain, but that’s what the best play-callers do on every single play. At least the good ones do. And sometimes all of that thinking doesn’t pay off. McDaniels tried to use the same tactic later in the drive, but this time the Lions motioned the slot corner over with Gronkowski, leaving Davis unblocked to make a play.

This is the chess match that takes place during games, and having learned under the best defensive coach of all-time, Patricia is a fairly decent chess player.

So far we’ve seen the Rams use a formation to get a certain coverage. We’ve also seen the Patriots use personnel and a pre-snap shift to create a mismatch. Now let’s look at an example where a play-caller uses all of those tools to create a touchdown play. For that, we go back to the Rams’ opening drive against the Vikings.

The Rams are in a three-receiver set and line up in an empty formation. Typically when an offense lines up in empty, the defense automatically checks to a certain coverage. In this instance, the Vikings check to a three-deep zone defense.

Take note of the safety in the deep middle cheating to the side with three receivers. McVay certainly did.

So here’s what McVay comes up with:

He knows by this point that if he runs 3 TEs onto the field, the Vikings will match by going to their base defense.

He also has figured out by now that if the Rams go to an empty backfield, the Vikings check to a three-deep zone.

In that zone coverage, he’s gleaned, the cornerbacks will stay on outside no matter who lines up there.

So McVay puts Robert Woods, named All-American in track by USA TODAY coming out of high school, in the slot to the two-receiver side. If the safety cheats over to the three-receiver side again, the Rams will have Woods going up against a linebacker with no safety help.

That’s exactly what happens. Let’s see how that matchup worked out for Minnesota.

McVay knew the Rams had a touchdown before the ball was even snapped. He had done his job as a play-caller. Jared Goff and Woods just had to do theirs.

After the game, Vikings coach Mike Zimmer ripped his defense, saying “we’ve never been this poor in pass coverage.” Zimmer can blame the players, but when Barr is left on an island against Gurley, Woods and Cooper Kupp, that’s just bad scheme. The Vikings defense wasn’t out-played Thursday night; it was out-coached. Zimmer fell into all the traps McVay knew he would fall into.

Good play-calling has little to do with drawing up intricate plays. Nor is it a “feel” thing, as it’s often described. It’s about making observations, gathering information and using all the tools at coach’s disposal to create favorable matchups based on that information.

The best play-callers aren’t artists. They’re scientists experimenting on defenses in their lab every Sunday.