A reference oscillator found in some synths, mostly modulars. It is a highly stable circuit designed to output a frequency of exactly 440 Hz, for use in tuning other synth circuits. Some types use heated and thermally stabilized components, and have to be left on for a period of minutes in order to stabilize. 440 Hz corresponds to concert A pitch (the A above middle C on a piano), which is the modern standard for most Western music.