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Cover Two is a popular type of zone defense used by teams in American Football. It involves the cornerbacks to cover the area out in the flat (the area about 5-10 yards deep to the left and right of the line of scrimmage) while the linebackers cover the middle of the field. This is what is called the underneath coverage. The safeties drop back deep and must cover their half of their field and about 30 yards of the field vertically.

Assumption: The players on defense are playing perfectly and know their role in the defense. They are also assumed to be playing in a standard 4-3 while the offense is going for the basic set (two running backs, two wide recievers, one tight end

Run situation: In this case the four linemen and three linebackers primary job is to contain the runner until more help arrives. What usually is the case is one of the linebackers or linemen break through the blocks and manages to tackle the runner himself.

When that does not happen, the primary goal is to either contain the running back towards the inside or push the running back laterally to the outside. Pushing the running back to the outside allows more help to arrive as the runner is forced to run parallel to the line of scrimmage. Keeping the running back on the inside prevents him from really going anywhere. The cornerbacks play a run as though it was a pass until the running back reaches that cornerback's side. The safeties are playing pass all the way until the running back passes the line of scrimmage. In which case the safety on the side of where the running back runs is in charge of containing the outside of the run path while the other safety attempts to stop the running back's cutback move to the inside.

Pass situation: In a passing play, the linemen rush while the linebackers drop back to play the pass. The linebacker's main job is to cover the area in the middle about 5 to 10 yards deep and to force the quarterback to throw over them to prevent deflections or interceptions. The corners must jam the wide reciever at the line of scrimmage otherwise it prevents the safety from effectively covering the deep pass. The corner will also attempt to force the reciever towards the inside, thereby allowing the safety to cover the deep pass better. The role of the safety is to protect his half of the field, he must be allowed to drop back about 12 yards (from the line of scrimmage) so he can cover any deep inside passes or help the corner on the outside in an intermediate pass.

The weakness to this type of defense rests in how well the cornerback jams the wide reciever. Good quarterbacks/wide recievers know when they are playing against this type of defense and attempt to find the "seam" between where the linebackers and safeties are playing "zone". The key to this defense working is how well (poorly) the quarterback reacts to the setup, the ability of the safety to get into the correct position and whether or not the cornerback got a good "jam" on the wide reciever.

i'm no football 'expert'. so if you see mistakes, like that huge glaring one, /msg me and tell me to fix it.
nice source of info -> http://www.webmail.com.au/~paul/2deep.htm