They are the farthest players away from each other on the football field. A team's two cornerbacks don't interact on the field much during a game, but each has an enormous effect on the other.
A shutdown corner on one side and a liability on the other does absolutely nothing for a defense.
Darrelle Revis with Lito Sheppard was nothing special.
Cortland Finnegan combined with Nick Harper was a disaster.
To truly have a dominant secondary, two talented cornerbacks must combine.
It's when you get the Charles Woodson-Al Harris combination a defense really gets scary.
Here are the rankings of the cornerback duos worst to best in the NFL this season: