C (Italian, French: Do) is the first note of the C major scale, the third note of the A minor scale (the relative minor of C major), and the fourth note (F, A, B, C) of the Guidonian hand, commonly pitched around 261.63 Hz. The actual frequency has depended on historical pitch standards, and for transposing instruments a distinction is made between written and sounding or concert pitch.

In English the term Do is used interchangeably with C only by adherents of fixed-Do solfège; in the movable Do system Do refers to the tonic of the prevailing key.

Historically, concert pitch has varied. 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). 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. After the A440 pitch standard was adopted by musicians, the Acoustical Society of America published new frequency tables for scientific use. A movement to restore the older A435 standard has used the banners "Verdi tuning", "philosophical pitch" or the easily confused scientific pitch.

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 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.

While the expression Middle C is generally clear across instruments and clefs, some musicians naturally use the term to refer to the C note in the middle of their specific instrument's range. C4 may be called Low C by someone playing a Western concert flute, which has a higher and narrower playing range than the piano, while C5 (523.251 Hz) would be Middle C. This technically inaccurate practice has led some pedagogues to encourage standardizing on C4 as the definitive Middle C in instructional materials across all instruments.^[1]

On the Grand Staff, middle-C is notated with a ledger line above the top line of the bass clef or below the bottom line of the treble clef.

In vocal music, the term Soprano C, sometimes called High C,^[2] is the C two octaves above Middle C. It is so named because it is considered the defining note of the soprano voice type. It is C6 in scientific pitch notation (1046.502 Hz) and c′′′ in Helmholtz notation. The term Tenor C is sometimes used in vocal music to refer to C5,^[3] as it is the highest required note in the standard tenor repertoire. Both notes, soprano and tenor, are also called Top C.^[4] The term Low C is sometimes used in vocal music to refer to C2 because this is considered the divide between true basses and bass-baritones: a basso can sing this note easily whereas other male voices, including bass-baritones, typically cannot.

Tenor C is an organ builder's term for small C or C3 (130.813 Hz), the note one octave below Middle C. In stoplists it usually means that a rank is not full compass, omitting the bottom octave.

(20,000 hertz is the start of the ultrasound in healthy young adults.)

Traversing the circle of fifths can result in a B♯ that is higher than C by 23.46 cents, the ratio of twelve just perfect fifths (B♯) to seven octaves being 531,441 / 524,288, the Pythagorean comma. A B♯ that is three just major thirds above C is lower than the octave by an interval called a diesis, 125:128 or 41.06 cents.