As easily as a thermometer tells you the temperature, KeyOmeter automatically determines and displays the musical key. KeyOmeter operates in the "circle of fifths" -- a visual representation of the 12 tonal centers or keys of the Western chromatic scale. The outer circle displays the twelve major keys, the inner circle shows their relative minors. Most music is not in a single key but modulates from one key to another. KeyOmeter follows and displays changes in key over time in live music.