The Steel Curtain defense of the 1970s was a pure 4-3, relying on its defensive linemen to stop the run and harass opposing quarterbacks into mistakes.
The linebackers were largely used in coverage in addition to cleaning things up in the running game.
But as the stars of that Steel Curtain defense began to retire, Chuck Noll was faced with a dilemma. Picking at the middle or lower in the draft for years and the Steelers' loss of their advantage of being one of the few teams to mine historically black colleges for players -- thanks to the scouting of Bill Nunn -- the team couldn't keep playing the same way it had.