The Rain Song" started as a folk piece.

Yes. That’s the one. It began life as a gentle acoustic thing Jimmy Page had lying around, and Robert Plant’s “The Rain Song” lyrics fit it. Then they took it to the band, added the Mellotron, the crescendo, the heavy middle section, and it became that epic eight-minute track on Houses of the Holy.

The folk-to-hard-rock transformation is exactly what made it work. It doesn’t just rock, it breathes. The quiet parts wouldn’t hit as hard if they weren’t originally conceived as something small.

If you ever want to hear the folk version, John Paul Jones’s Mellotron arrangement is basically the bridge. But the original idea was all Page and an acoustic guitar.