The decimal number, from 0-128, which represents the equal temperament scale of about eight octaves, where 60 represents Middle-C, having a frequency of 261.63Hz. The MIDI note number 36, for example, corresponds to the 4th key on a piano, referred to as C1, with a frequency of 32.7Hz. Middle-C is sometimes called C3 or C4, depending on the author. Commonly, modern instruments are tuned to A440, that is A3/A4, MIDI note number 69.