A cornerback (CB) (also referred to as a corner) is a positional role played by a member of the defensive team in American and Canadian football. A cornerback is a member of the defensive backfield or secondary, which also may include the safety and linebacker positions.
The rules of American professional football (NFL Rulebook) and American college football (NCAA) do not specify starting position, movement, or coverage zones for members of the defensive team, except that they must be in the defensive area or zone at the start of play. The positions, movements and responsibilities of all defensive players are assigned by the team. The positional roles are therefore customary, not assigned by rule but by patterns of coverage assigned by the coach or defensive captain before the play begins. These roles have varied over the history of American football. The principal defensive responsibility of the cornerback is to legally interfere with or intercept a forward pass attempt made by the offense. Other assignments may be to tackle a ball carrier from the opposing team carrying the ball, or to blitz, or enter the offensive backfield after the start of play in an attempt to tackle a ball carrier on the offensive team.
The rules to how one may go about this are different at different levels of play. For example, in the professional level, a cornerback is not allowed to make any contact with an eligible receiver after five yards; whereas any level under professionals can maintain some contact in order to know where the receiver is. If a cornerback is judged to violate these rules of contact with a receiver, the penalty they receive is most commonly pass interference depending on the extent to which the rule is violated.
The role of a cornerback (or safety) in coverage is to act as a "shadow receiver" for forward passes. He is assigned either to a specific offensive receiver (in man-to-man coverage) or within a certain zone of the defensive playing field (zone coverage.) The rules of legal coverage allow both offensive receivers and defensive coverage to have a fair and unimpeded chance at catching a thrown ball. A defensive player in coverage may disrupt the ball in flight or during an attempted catch, but may not push or strike the opposing player in order to impede his ability to catch the ball. Violation of the rules by either the offensive or defensive player is referred to as pass interference and the offending team is assessed a penalty.
As a critical element of any defensive secondary, the cornerback position demands players have speed, agility, and exceptional instincts. Additionally, a cornerback's skillset typically requires proficiency in backpedaling, shedding blockers, man-to-man coverage, disrupting pass routes, jamming receivers at the line of scrimmage, reading the quarterback, understanding zone coverage, and open-field tackling.
Most modern NFL defensive formations use four defensive backs (two safeties and two corners); CFL defenses generally use five defensive backs (one safety, two defensive halfbacks, and two corners). A corner's responsibilities vary depending on how the defense assigns protection to its defensive secondary to handle the offense. In terms of defending passing plays, a corner will be typically assigned to either zone or man-to-man coverage.