The defensive line is responsible for:
IMPORTANT: There is also a left tackle position in the offensive line, but do not confuse the two.
IMPORTANT: There is also a right tackle position in the offensive line, but do not confuse the two.
The defensive end stands both ends of the defensive line. They stand opposite the offensive tackles. Defensive ends have the best chance to sack the quarterback because they are close to the sidelines.
Defensive coaches use a combination of lettering and numbering systems to describe specific "gaps" between the offensive linemen, as well as how defensive linemen will line up in relation to the offensive linemen across from them.
The A gaps are located in between the center and guards. The B gaps are between the guards and offensive tackles. The C gaps are between the offensive tackle(s) and tight end(s), and the D gap is outside the tight end.
For defensive line techniques, even numbers are for "head-up" positions (when the defender lines up directly across from the offensive lineman) starting at 0 across from the center and ascending order to the outside. Odd numbers are for alignments (or "shades") to the outside shoulder of the offensive player. There also inside-shoulder techniques, which use even numbers followed by an "i" designation (because the defender is lining up even with the offensive lineman's eye).
Gap and technique codes are used to quickly tell players where to line up and where to go when the play begins in a timely manner. For example, in a 4-3 Over defense, the strong-side defensive tackle lines up in a 3-technique and plugs the B-gap.