Overview
The chief responsibility of the cornerback is to defend against the offense's pass. The rules of American professional football and American college football do not mandate starting position, movement, or coverage zones for any member of the defense. There are no "illegal defense" formations. Cornerbacks can be anywhere on the defensive side of the line of scrimmage at the start of play, although their proximity, formations, and strategies are outlined by the coaching staff or captain.
Most modern National Football League defensive formations use four defensive backs (two safeties and two corners); Canadian Football League defenses generally use five defensive backs (one safety, two defensive halfbacks, and two corners). A cornerback's responsibilities vary depending on how the defense assigns protection to its defensive secondary. In terms of defending the run, oftentimes corners may be assigned to blitz depending on the coaching decisions in a game. In terms of defending passing plays, a corner will be typically assigned to either zone or man-to-man coverage.
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