The riff, while written while the band was Turing, is brutally simple and effective. The riff itself only takes 2.7 seconds to play, but it immediately projected music into another decade. While everyone else was still playing in the 60s, zeppelin were now playing in the 1970s. In my opinion, this riff edges out Randy Rhodes opening salvo to Ozzy, Osborne's, Crazy Train, and the solid as concrete, instantly recognizable riff that opens AC DC's back in Black for a piece of history