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Shutdown cornerbacks still exist; they're just harder to identify in today's pass-happy NFL.

So many yards are accumulated through the air that it makes the term "shutdown" anachronistic, or at least relative to how the game is currently played.

The days of Deion Sanders no longer exist. Cornerbacks aren't completely shutting down one side of the field and being avoided like the plague by opposing quarterbacks. The game is geared toward offense after years of rule changes favoring passing attacks.

What actually defines a shutdown corner when the game is stacked against the position?

The Denver Broncos' Chris Harris Jr. provided a description for NFL Media's Bucky Brooks:

Guy who's not giving up touchdowns. I think in this league, those "tubs" [touchdowns] are the games that you win or lose. And if you ain't giving up touchdowns, you're doing whatever you can for your team to win that game. Also, in those big, key third downs -- those big, key plays -- you gotta come up and make those plays. And I think the shutdown corners are making those plays most of the time.

Harris views himself as a true shutdown corner, and he's certainly one of the league's best. The Broncos defensive back experienced a stretch of 35 games between the 2013 and '15 seasons when he didn't surrender a score. He officially gave up two touchdowns by the end of the 2015 campaign, though.

The formula to determine a true shutdown corner must expand beyond how many touchdowns are allowed, because 14 cornerbacks gave up two or fewer scores last season. Clearly, there aren't 14 different corners playing today who can be termed "shutdowns."

Preventing the other team from scoring is certainly important, but it's only part of the equation. A true shutdown corner should also discourage quarterbacks from throwing in his direction. It should also be more difficult to complete a pass when that defensive back is in coverage. He needs to be the entire package, even when everything is slanted to the wide receiver's advantage.

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This is where it becomes tricky to identify the best of the best.

Along with Harris, the Arizona Cardinals' Patrick Peterson and even an aging Darrelle Revis remain the league's best cover corners.

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"At the end of the day, I know I'm the best," Peterson said, per USA Today's Lorenzo Reyes. "I know I can get in the face of the receiver, and I mean much more to my team versus those guys to their team."

The advanced statistics back up Peterson's claim.

Last season, quarterbacks completed a pass against the Cardinals defensive back once every 19.5 plays—the highest percentage of targets between receptions. Peterson finished first by surrendering only 0.58 yards per coverage snap and tied for second with 9.3 coverage snaps per target. Quarterbacks only completed 47.7 percent of passes thrown toward his coverage, which equates to a 61.8 quarterback rating.

No other cornerback finished among the top six performers in all five categories. A handful came close but didn't quite make the cut in one or more categories.

For example, Revis ranked sixth in coverage snaps per reception (14.0), 12th in yards per coverage snap (1.04), 21st in coverage snaps per target (6.5), third in opposing quarterback rating (56.5) and first in percentage caught when in coverage (46.5).

The Jets cornerback put together a stellar season for a 30-year-old defensive back. While his overall athleticism will start to decline with age, Revis should continue to play at a high level due to his preparation, work ethic and unparalleled technique.

Harrison, Peterson and Revis are generally asked to do more than most, too. They can be found lined up in multiple positions and tasked with shutting down a No. 1 target on a weekly basis.

This is where the conversation can often deteriorate due to how other top corners are utilized. The system influences a player's effectiveness, which in turn affects perception.

Zone corners are viewed less favorably than man corners who lock down receivers.

"I've talked to DT [Demaryius Thomas], I've talked to guys on our team that say, 'This man follows me the whole game and plays zone. But I can't consider them a shutdown corner,'" Harris told Brooks. "So coming from the receivers, they don't consider you a shutdown corner unless you're playing man."

Obviously, this is a very small sample size, but the argument often comes up when comparing Peterson to the Seattle Seahawks' Richard Sherman, who is also considered a premier cornerback. Why? Because Sherman only lines up on one side of the field and benefits from his Legion of Boom teammates and the team's vaunted Cover 3 scheme.

Peterson's former teammate, Antonio Cromartie, called Sherman out last summer for not being asked to do more in Seattle's system, per SiriusXM NFL Radio:

Go play in a defense where you don't have two All-Pro safeties. ... Go follow the No. 1 receiver. Follow him around for a whole entire game and see what you can do. Darrelle Revis has done that his whole career. I've done that, Patrick Peterson has done it, Joe Haden has done it. He's the only defensive back that hasn't. There's no point in critiquing him. If you want to label yourself as the No. 1 corner, the best corner in the NFL, follow the best guy on every single team. His whole thing will be "we don't have to do that, I got my trust in the other corner on the other side." That's not the point. If you want to consider yourself the best, the best do what the best do and they follow around the best. And that's what we're all getting to.

In Sherman's defense, he can only perform in the role asked of him, and he excels. Last year, the outspoken defensive back finished top-four in coverage snaps per reception (19.2), coverage snaps per target (9.3), yards per coverage snap (0.73) and percentage caught in his coverage (48.4).

Plus, the Seahawks' left corner displays some of the league's best ball skills with 22 interceptions and 62 deflected passes during the past four seasons.

The concern is whether he'd be less effective in an expanded role. An interesting case study can be found in Atlanta with Falcons cornerback Desmond Trufant.

The former first-round pick is one of the game's best young cornerbacks. He improved in each of his first three seasons. But his improvement last year might be due in part to playing in the same scheme as Seattle's after former defensive coordinator Dan Quinn took over as head coach.

Desmond Trufant's performance (2014 vs. '15) Year QB rating Cover snaps/target Yards/cover snaps Cover snaps/reception Comp. % 2014 74.0 6.5 1.08 11.1 58.7 2015 86.5 9.8 0.79 17.1 57.1 Pro Football Focus

A scheme change was the only significant difference in the Falcons defense last year, particularly in the secondary. With Quinn at the helm, Atlanta improved from 32nd overall in total defense to 16th last year. Scheme certainly played a major part.

A heavy zone system isn't as reliant on top secondary talent. Harris explained why it's easier to play compared to man, per Brooks:

The routes that you have to cover are limited. Say you're in Cover 2, then you're just jamming and you're sinking under the corner routes. So you're playing those two routes. Playing Cover 3, you're just playing your deep third, or you're playing the "bang" [skinny post] routes. So you really get limited routes that you have to cover, so it makes it easier for you. But if you're playing man, playing man in the slot, playing man outside, you got tons of routes that you gotta cover. And that's what makes it harder to be a man corner.

This is why a successful organization like the Carolina Panthers allowed cornerback Josh Norman to leave after a career year in which he received All-Pro and Pro Bowl honors.

Carolina's defense is built front-to-back with a heavy emphasis on zone coverage. Norman will now have to prove himself with the Washington Redskins after signing a five-year, $75 million contract.

In Washington, Norman will be asked to do more. He's excited about the opportunity after playing in a highly structured scheme, per the MMQB's Emily Kaplan.

"In Carolina everything was defined. It was black and white and some gray," Norman said. "Here everything is gray—it's more free. I can not just be in a scheme, but be a football player. I have a little more freedom. The shackles have been broken off, and man, I'm going to show them something they've never seen before."

Another performance like last year will place Norman in the conversation as a shutdown corner.

"He's an up-and-coming guy," Peterson told Reyes.

Until then, those defensive backs who deserve the designation can be counted on one hand when factoring in Harris' parameters, advanced statistics and schematic advantages.

Based on those criteria, Peterson backed up his claim that he's the NFL's best cornerback. Revis and Harris may be a step behind, but they certainly deserve recognition as top cover men.

The trio represent the NFL's modern version of a shutdown corner.

All advanced statistics courtesy of Pro Football Focus. Salary information via Spotrac. Brent Sobleski covers the NFL for Bleacher Report. Follow him on Twitter @brentsobleski.