sentences:  "Over" and "under" defenses simply refer to the shift of the defensive linemen to the strong or weak side of the offense, respectively, and the rotation of the linebackers in the opposite direction.The "Fairbanks-Bullough" 3–4 system is known as a two gap system, because each of the defensive linemen are required to cover the gaps to both sides of the offensive lineman that try to block them.Defensive linemen in this system tend to be stouter, as they need to be able to hold their place without being overwhelmed in order to allow the linebackers behind them to make plays.