Led Zeppelin's fourth album, Black Sabbath's Paranoid, and Deep Purple's Machine Head have stood the test of time as the Holy Trinity of English hard rock and heavy metal, serving as the fundamental blueprints followed by virtually every heavy rock & roll band since the early '70s.

And, though it is probably the least celebrated of the three, Machine Head contains the "mother of all guitar riffs" -- and one of the first learned by every beginning guitarist -- in "Smoke on the Water." Inspired by real-life events in Montreux, Switzerland, where Deep Purple were recording the album when the Montreux Casino was burned to the ground during a Frank Zappa concert, neither the song, nor its timeless riff, should need any further description.

However, Machine Head was anything but a one-trick pony, introducing the bona fide classic opener "Highway Star," which epitomized all of Deep Purple's intensity and versatility while featuring perhaps the greatest soloing duel ever between guitarist Ritchie Blackmore and organist Jon Lord.

Also in top form was singer Ian Gillan, who crooned and exploded with amazing power and range throughout to establish himself once and for all as one of the finest voices of his generation, bar none.

Yes, the plodding shuffle of "Maybe I'm a Leo" shows some signs of age, but punchy singles "Pictures of Home" and "Never Before" remain as vital as ever, displaying Purple at their melodic best.

And finally, the spectacular "Space Truckin'" drove Machine Head home with yet another tremendous Blackmore riff, providing a fitting conclusion to one of the essential hard rock albums of all time.