<P> A cornerback (CB), also referred to as a corner, is a member of the defensive backfield or secondary in American and Canadian football . Cornerbacks cover receivers, to defend against pass offenses and make tackles . Other members of the defensive backfield include the safeties and occasionally linebackers . The cornerback position requires speed and agility . A cornerback's skillset typically requires proficiency in anticipating the quarterback, back - pedalling, executing single and zone coverage, disrupting pass routes, block shedding, and tackling . Cornerbacks are among the fastest players on the field . </P> <P> 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 . </P>

What does a cornerback do in american football