 Middle C (the fourth C key from left on a standard 88-key piano keyboard) is designated C4 in scientific pitch notation, the most commonly recognized notation in auditory science, while both C4 and the Helmholtz designation c′ are used in musical studies.Other note-octave systems, including those used by some makers of digital music keyboards, may refer to Middle C differently.In MIDI, Middle C is note number 60 which equates to C5.