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three-line

Under the Helmholtz Octave Designation System, a name applied to the octave which extends from C3  in the octave above the treble clef staff to B3 in the second octave above the treble clef staff. The notes are indicated by lowercase letters with the superscript "3" (c3, d3, e3, f3, g3, a3, and b3).
 
This octave can also be designated with three hash marks (i.e. C''') or in the 6th octave with Scientific Pitch Notation (spn) using uppercase letters with octave "6" (C6, D6, E6, F6, G6, A6, and B6). This is also known as american_standard_pitch_notation (aspn) and was defined by the Acoustical Society of America.
 
The MIDI Octave Designation System has two versions, one where note 1 (8.176 Hz) is octave "0" making the Three-line octave the seventh octave (e.g. C7), and the other with note 1 (8.176 Hz) at octave "-2" making the Three-line octave the fifth octave (e.g. C5).
 
The indication 8va (shown below) can be used to represent the same pitches without as many ledger lines.
 
See the table of octave designations in the Appendix.