Anyone who frequented record shops in the 1970s will recall the term ‘File Under Progressive’, an instruction found on the sleeves of LPs by those groups that rarely, if ever, turned up on Top Of The Pops. Foremost among these rarified bands was Led Zeppelin, a group that never appeared on TV, released singles or even bothered to include their name on most of their LP covers.

These days ‘progressive rock’ is a term applied to Zeppelin’s musically ambitious contemporaries – the likes of Emerson, Lake & Palmer, Genesis or Yes – and modern acts inspired by those 70s bands. In truth, Led Zeppelin were more ‘progressive’ than any of them.

In 1967, The Beatles’ Sgt Pepper’s Lonely Hearts Club Band LP stuck a marker in the sand. From here on, many groups began thinking beyond the confines of the three-minute pop single. Among these were future Zeppelin guitarist Jimmy Page’s band The Yardbirds, whose epic track Dazed And Confused featured Page conjuring unearthly sounds by scraping a violin bow across the strings of his guitar. ‘Pop’ had acquired a hairier, smellier older brother called ‘rock’, and all bets were off.

A new version of Dazed And Confused became the centerpiece of Led Zeppelin’s 1969 debut album. Although Zeppelin’s music was always rooted in the blues, the band, especially Page and bassist/keyboard player John Paul Jones, would spend the next 10 years and seven albums pushing the boundaries.