All 3-4 fronts are not the same, however. While the 3-4 front is traditionally thought of as a 2-gap front, there are two major families of 3-4 in use across the league today. ** But the 2-gap 3-4 front is more difficult to play in todays N.F.L. Those planet-like defensive linemen are getting harder and harder to find. ** As a result, a majority of the 3-4 fronts today are based on the 1-gap schemes designed by Bum Phillips or those that use other wrinkles to bring pressure and disguise coverage. Other than Parcells Dallas and Miami teams in recent seasons, every other contemporary 3-4 defense mixes in 1-gap techniques liberally, some exclusively. The true 3-4 front has become a dinosaur of sorts as an every-down defense. ** That attacking style of play has stood the test of time better than the read-and-react style for much the same reason that the 4-3 with an under or over shift has. It allows players to attack the offense, specifically by disguising the defenses fourth (and fifth or sixth) pass rusher and the coverage behind. In fact, there are a lot of under front concepts in the Phillips 3-4. http://fifthdown.blogs.nytimes.com/2010/09/09/guide-to-n-f-l-defenses-part-4-the-3-4-front/ && This is a very lengthy article in the NY Times, but it not only lays out the different "families" of 3-4s, it describes the history and development and evolution of the defense by many coachs including Bum Phillips and Bill Parcells.