This octave can also be designated with three uppercase letters (i.e. CCC) or in the fundamental octave under Acoustical Society of America system (i.e. C0). The MIDI Octave Designation System has two versions, one where note 1 (8.176 Hz) is octave "0" making the Sub-contra octave the first octave (i.e. C1), and the other with note 1 (8.176 Hz) at octave "-2" making the Sub-contra octave the negative 1 octave (i.e. C-1).
The indications 8vb and 15mb (shown below) can be used to represent the same pitches without as many ledger lines. The indication 15mb is rarely used today.
Play Sub-contra Octave Example (MIDI file format)