Sub-contra

Under the Helmholtz Octave Designation System, a name applied to the octave which extends from C 2 three octaves below the bass clef staff to B 2 two octaves below the bass clef staff. The notes are indicated by uppercase letters with the subscript "2" ( C2, D 2, E 2, F 2, G 2, A 2, and B 2). 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. C 0). 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.