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. This is the reason that defensive linemen such as Richard Seymour and Vince Wilfork do not always rack up sack and tackle statistics despite their critical importance to the team.