This octave can also be designated with four hash marks (i.e. C'''') or in the 7th octave under Acoustical Society of America system (i.e. C7). The MIDI Octave Designation System has two versions, one where note 1 (8.176 Hz) is octave "0" making the Four-line octave the eighth octave (i.e. C8), and the other with note 1 (8.176 Hz) at octave "-1" making the Four-line octave the sixth octave (i.e. C6).
The indications 8va and 15ma (shown below) can be used to represent the same pitches without as many ledger lines. The indication 15ma is rarely used today.
Play Four-line Octave Example (MIDI file format)