Under the Helmholtz Octave Designation System, a name applied to the octave which extends from C 2 in the middle of the treble clef staff to B 2 above the treble clef staff. The notes are indicated by lowercase letters with the superscript "2" ( c2, d 2, e 2, f 2, g 2, a 2, and b 2). 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. C 5). 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.