Steve Reich’s piece Piano Phase involves two pianos playing the same melody simultaneously at slightly different speeds.
I am informed by people in the know that this is an absurdly difficult piano piece to play. Piano Phase, composed in 1967, begins with a nice little twelve note melody. That part is played over and over on two pianos. At first they are in perfect unison, but then one of the pianists very gradually increases their tempo. Ever so gradually, the two parts move out of sync.
The effect is very disconcerting: like an echo at first, building into a messily intricate interweaving of notes, and then eventually moving back into sync with each other for a simple and unified climax. It’s minimalist and hypnotic and unsettling and beautiful. It’s also a bit of a tough listen if you’re not familiar with this kind of art music, so maybe dip your toes in first?
Steve Reich, the American composer of this and several other “phase music” pieces, began his experiments with just one piano – playing the first part, recording it, and then playing the second over the top of that recording – but modern performances often use two players and two pianos:
Or, incredibly, one player and two pianos: