It's the Mellotron, not just a keyboard.

The Mellotron. John Paul Jones used a Mellotron on that track—the tape-based keyboard that gave ‘No Quarter’ its eerie, choral, almost orchestral feel. That’s the iconic sound.

Everyone remembers the piano part too, but the Mellotron is what makes the song float. It’s that warbly, slightly haunted backdrop that sounds like a choir from the bottom of a well. Jones rented it specifically for the session because he wanted something other than a plain organ or piano.

You can hear it most clearly in the verses and the long instrumental sections. It’s not a synth, it’s not a Hammond—it’s a Mellotron. And that’s why the song still sounds like nothing else.

If you’re aiming for that exact tone, a digital Mellotron emulation will get you 95% of the way there without buying a vintage tape machine.