Defensive tackles

A defensive tackle (DT) is typically the largest and strongest of the defensive players in American football. The defensive tackle typically lines up opposite one of the offensive guards. Depending on a team's individual defensive scheme, a defensive tackle may be called upon to fill several different roles. These roles may include merely holding the point of attack by refusing to be moved or penetrating a certain gap between offensive linemen to break up a play in the opponent's backfield. If a defensive tackle reads a pass play, his primary responsibility is to pursue the quarterback, or simply knock the pass down at the line if it's within arm's reach. Other responsibilities of the defensive tackle may be to pursue the screen pass or drop into coverage in a zone blitz scheme. In a traditional 4–3 defensive set, there is no nose tackle. Instead there is a left and right defensive tackle. Some teams especially in the NFL do have a nose tackle in this scheme, but most of them do not.

Nose tackle (also nose guard or middle guard) is a defensive alignment position for a defensive lineman. In the 3–4 defensive scheme the sole defensive tackle is referred to as the nose tackle. The nose tackle aligns across the line of scrimmage from the offense's center before the play begins in the "0-technique" position. In this position, frequently taking on the center and at least one if not both of the guards, the nose tackle is considered to be the most physically demanding position in football. In five-linemen situations, such as a goal-line formation, the nose guard is the innermost lineman, flanked on either side by a defensive tackle or defensive end. According to Pat Kirwan, a traditional 3–4 defense demands "a massive man who can clog up the middle," while a 4–3 defense is looking for "a nose tackle who relies on quickness to penetrate and move along the front."

...
Wikipedia