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For the most part, the lifespan of a true shutdown cornerback in the NFL is unmercifully short.

Had this article been written three years ago, we'd likely be discussing the upcoming 2016 performances of Richard Sherman, Patrick Peterson, Josh Norman and Darrelle Revis. Now, none of those players are near the top of the list.

The position is difficult to attain because of the traits of a shutdown corner, and that's why even the best in the business struggle to maintain their status. A shutdown corner has to erase his opponent's best receiver on a week-to-week basis, which requires him to adjust his game to that player.

One week, he may be facing a 6'5", 240-pound behemoth who wants to jar his helmet from his head on every down. The next, he could be covering a 5'10" track star who could tie him knots with perfectly run routes.

Not only does he have to understand and execute coverage in zone, man and hybrid concepts, but he must also take advantage of specific techniques that allow him to turn receptions into disasters for the opposing quarterback.

He must play the boundary all the way down the field to take away the vertical routes. He must do the same when his receiver is headed up the seam as quickly as possible. He may need to slide into the slot if that's where his receiver goes, and then he's covering his man without the sideline as a helper. He must play trail, bail and press coverage at a high level, and he must do it over and over, more than 1,000 times per season.

It's no mystery why shutdown cornerbacks are so valuable in today's game. Here are the 10 best heading into the 2018 NFL season.