Low C
In terms of musical pitch, C, often referred to as Do, is the first note of the fixed-Do solfège scale.
The use of the term Do to refer to C is not universal. While C refers to a specific pitch (usually 261.63 Hz), Do is used by some authors to the tonic note of a musical key. In other words, depending on the author Do may vary with the key of the music whereas C is always a fixed pitch.
When the A440 pitch standard is used to tune a musical instrument, Middle C has a frequency around 261.63 Hz. Middle C is designated C4 in scientific pitch notation because of the note's position as the fourth C key from left on a standard 88-key piano keyboard.
Another system known as scientific pitch assigns a frequency of 256 Hz but, while numerically convenient, this is not used by orchestras. 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.
The C4 designation is the most commonly recognized in auditory science, and in musical studies it is often used in place of the Helmholtz designation 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. 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.
In vocal music, the term Soprano C, sometimes called High C or Top C, 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, as it is the highest required note in the standard tenor repertoire. 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 while other male voices, including bass-baritones, cannot.
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