In November 1971, we unwrapped the new Led Zeppelin album. Side one, track four, went on to become the band’s defining song, an enduring international favourite, and one of the most heard and covered songs ever.

It’s not Led Zeppelin’s best song. It’s not even the best song on the album. But it is a magical thing, a gradually unfolding musical odyssey building from a gentle guitar and recorder duet, to a thrilling, almost symphonic crescendo. Jimmy Page refused to cut it down for a 45rpm release, but radio played it anyway and the song became one of rock’s biggest hits.

Page composed the music and the extraordinary guitar parts, to which whole websites and YouTube channels are devoted. Robert Plant’s enchanting lyrics are also pored over and analysed for meaning. Because you know sometimes words have two meanings.