In vocal music, the term High C (sometimes less ambiguously called Top C ) can refer to either the soprano's C 6 (1046.502 Hz c′′′ in Helmholtz notation) or the tenor's C 5 both are written as the C two ledger lines above the treble clef but the tenor voice sings an octave lower. Alternatively, it is written on the centre line of a staff using the alto clef, or on the fourth line from the bottom, or the second line from the top, of staves using the tenor clef. On the Grand Staff, middle-C is notated with a ledger line above the top line of the bass staff or below the bottom line of the treble staff.

This technically inaccurate practice has led some pedagogues to encourage standardizing on C 4 as the definitive Middle C in instructional materials across all instruments. C 4 may be called Low C by someone playing a Western concert flute, which has a higher and narrower playing range than the piano, while C 5 (523.251 Hz) would be Middle C. 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. 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).

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