The 46 Defense was made famous by Buddy Ryan and the Super Bowl XX champion 1985 Chicago Bears. Unlike most other defenses, the 46 is not named for the alignment of the DL and LBs. The 46 was named after Chicago Bears hard-hitting safety Doug Plank who wore number 46. The 46 defense is still seen in the NFL today. Rex Ryan runs it with the New York Jets and previously when he was the defensive coordinator of the Baltimore Ravens.
The original 46 Defense was designed to utilize personnel from the 4-3. In recent years, coaches have modified the defensive scheme in order to run it from whatever their base personnel is. Some will run it from a 44 Defense while it seems to naturally fit best into a 3-4 Defense or 3-3 Stack Defense.