Four sound sources, one joystick. Vector synthesis (the Prophet VS, the Korg Wavestation) puts a source at each corner of a square and mixes them by where a point sits inside it — drag toward a corner and you hear that source; sit in the middle and you hear all four at once. Let the point travel on its own and the timbre moves, which is the whole appeal: an evolving pad from four simple waveforms. The mix is a true crossfade (the four weights always sum to one), so it never clips as you move.
play a note (a s d f …) and drag the pad — or switch on auto-motion and just hold a chord