The voice of Robert Plant is probably the most immediately identifiable element of any Led Zeppelin song. Whether it’s the throes of passion that open ‘Black Dog’, the shrieking cries that provide the hook for ‘Immigrant Song’, or the furious scat-singing in the breakdown of ‘Babe I’m Gonna Leave You’, or the sexually-charged sounds punctuating ‘Dazed and Confused’ and ‘Whole Lotta Love’, Plant’s rasping falsetto is unmistakable.

At the time of Zeppelin’s rise to prominence, it was unique in the history of rock and roll that someone’s dominant mode of vocal expression would be an octave or two above their natural singing voice yet filled with such grit and menace. But Plant’s vocal style opened the floodgates, shaping the sound of everyone from AC/DC to Wolfmother and Greta Van Fleet. By the time the group disbanded, the influence of Zeppelin’s singer on the future of rock and metal was already beyond question.

His vocal range also negated the need for a female singer even when melodies soared far above the reaches of most male voices. Singing on a Led Zeppelin record was reserved strictly for Plant. That is, except for one solitary song. “It was really more of a playlet, rather than a song,” he told Roy Carr in a 1971 interview. “After I wrote the lyric, I realised I needed a completely different voice, as well as my own.” And so, he approached the frontwoman of a band Zeppelin were friends with at the start of the ‘70s.

Fairport Convention were a well-established British folk band from whom Plant and Jimmy Page drew inspiration for the acoustic-driven elements of their music. And their singer was one of the country’s most prominent female vocalists during the latter part of the previous decade. “I asked Sandy Denny along to sing on that track,” Plant said. “I found it very satisfying to sing with someone who has a completely different style to my own.”

So, what was the song she sang on?
It was a session for Led Zeppelin fourth album that Denny was invited to join. She sang on the epic high fantasy-themed composition ‘The Battle of Evermore’, in which Plant narrates the story of a mythical castle siege based on Lord of the Rings. Backed by Page’s medieval mandolin, he and Denny joust to tell the story from opposing perspectives. Denny springs forth at the start of the second verse with an ethereal counter-melody that rises in notes of sustained purity, complementing Plant’s more prosaic and staggered delivery.

“When I sang about the events of the song,” Plant added, “Sandy answered back as if she was the pulse of the people in the battlements.” This back-and-forth dialogue helps give the track its sense of grandeur, as though it’s an epic poem told through the voice of multiple protagonists. “Sandy was a town crier urging the people to lay down their weapons,” its writer observed.

While Zeppelin would never repeat this experiment of having a female vocalist duet with Plant, there’s no doubt that it works a treat in ‘The Battle of Evermore’. And it’s one of the aspects of Led Zeppelin IV which sets it apart from the rest of the band’s canon, confirming it as the most creative and far-reaching work they ever produced.