The A-151 is a 4-stage sequential switch that advances to the next stage with each incoming trigger or clock pulse. It operates like a rotating switch, cycling through either four outputs from a single input, or vice versa. The signal path is bidirectional, allowing for routing of audio or CV in either direction. This makes it a versatile tool for evolving sequences, modulation routing, or switching between multiple sources. A compact but powerful utility in any Eurorack setup.