Middle C (the fourth C key from left on a standard 88-key piano keyboard) is designated C 4 in scientific pitch notation, and c ′ in Helmholtz pitch notation it is note number 60 in MIDI notation.

A movement to restore the older A435 standard has used the banners 'Verdi tuning', 'philosophical pitch' or the easily confused scientific pitch. After the A440 pitch standard was adopted by musicians, the Acoustical Society of America published new frequency tables for scientific use. Scientific pitch was originally proposed in 1713 by French physicist Joseph Sauveur and based on the numerically convenient frequency of 256 Hz for middle C, all C's being powers of two. For an instrument in equal temperament tuned to the A440 pitch standard widely adopted in 1939, middle C has a frequency around 261.63 Hz (for other notes see piano key frequencies). In English the term Do is used interchangeably with C only in the context of fixed Do solfège in the movable Do system Do refers to the tonic of the prevailing key. It has enharmonic equivalents of B ♯ and D.

The actual frequency has depended on historical pitch standards, and for transposing instruments a distinction is made between written and sounding or concert pitch.

C or Do is the first note and semitone of the C major scale, the third note of the A minor scale (the relative minor of C major), and the fourth note (G, A, B, C) of the Guidonian hand, commonly pitched around 261.63 Hz.

Middle c position