The New England Patriots run a modified base 3–4 Chuck Fairbanks-Hank Bullough system installed by Bill Belichick. The term 3–4 means that their base formation consists of 3 defensive linemen (defensive end, nose tackle, and defensive end), 4 linebackers (outside "Will" weak side linebacker, middle "Jack" weak side linebacker, middle "Mike" strong side linebacker, and outside "Sam" strong side linebacker), and 4 defensive backs (cornerback, free safety, strong safety, and cornerback). The theory of the system is that a 3–4 structure gives the defense the greatest amount of flexibility because the linebackers are versatile players on defense, capable of rushing the quarterback, tackling runners or dropping into coverage. By mixing the roles of their linebackers from play to play, the Patriot's defense seeks to cause confusion on the part of opposing offenses. At times the Patriots will also shade their defensive linemen different ways, creating "over" or "under" defenses. "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.