Roger Waters spat on a fan. Montreal, July 1977, the last night of a tour he’d grown to hate, crowds he was sure weren’t really listening. A fan near the stage got under his skin. Waters leaned over and spat in his face, then stood there, disgusted with himself. That gap between the man on stage and the bodies in the dark became the idea: build a wall across the stage, brick by brick, until the band disappears behind it.

That’s where The Wall starts. Released in 1979, it’s a double album about Pink, part Waters, part Syd Barrett, a rock star who seals himself off from the world one trauma at a time, then tears the wall down in the end. Bob Ezrin came in to shape the sprawl into something that moved. On the road, Floyd built the thing for real: a 160-foot barrier of cardboard bricks, raised across the stage and demolished in the finale, played just 31 times in four cities.

Thirty million copies later, it’s the best-selling double album ever made. Start at the bottom and climb.