This octave can also be designated with two hash marks (i.e. C'') or in the fifth octave under Acoustical Society of America system (i.e. C5). The MIDI Octave Designation System has two versions, one where note 1 (8.176 Hz) is octave "0" making the Two-line octave the sixth octave (i.e. C6), and the other with note 1 (8.176 Hz) at octave "-2" making the Two-line octave the fourth octave (i.e. C4).
The indication 8va (shown below) can be used to represent the same pitches.
Play Two-line Octave Example (MIDI file format)