octave in music theory encompasses chromatic alterations within the pitch class, meaning that G to G (13 semitones higher) is an Augmented octave (A8), and G to G (11 semitones higher) is a diminished octave (d8). The use of such intervals is rare, as there is frequently a preferable enharmonically-equivalent notation available (minor ninth and major seventh respectively), but these categories of octaves must be acknowledged in any full understanding of the role and meaning of octaves more generally in music. Notation Octave of a pitch Octaves are identified with various naming systems. Among the most common are the scientific, Helmholtz, organ pipe, and MIDI note systems. In scientific pitch notation, a specific octave is indicated by a numerical subscript number after note name. In this notation, middle C is C4, because of the note's position as the fourth C key on a standard 88-key piano keyboard, while the C an octave higher is C5. {| style="border-spacing: 2px; border: 1px solid darkgray;" ! style="text-align: right" colspan="13" | |- ! style="text-align: right" | Scientific | style="text-align: center; width: 67px;" | C−1 | style="text-align: center; width: 67px;" | C0 | style="text-align: center; width: 67px;" | C1 | style="text-align: center; width: 52px;" | C2 | style="text-align: center; width: 52px;" | C3 | style="text-align: center; width: 55px;" | C4 | style="text-align: center; width: 55px;" | C5 | style="text-align: center; width: 55px;" | C6 | style="text-align: center; width: 67px;" | C7 | style="text-align: center; width: 67px;" | C8 | style="text-align: center; width: 67px;" | C9 | style="text-align: center; width: 25px;" | |- ! style="text-align: right" | Helmholtz | style="text-align: center" | C,,, | style="text-align: center" | C,, | style="text-align: center" | C, | style="text-align: center" | C | style="text-align: center" | c | style="text-align: center" | c' | style="text-align: center" | c'' | style="text-align: center" | c''' | style="text-align: center" | c'''' | style="text-align: center" | c''''' | style="text-align: center" | c'''''' | |- ! style="text-align: right" | Organ | style="text-align: center" | 64 Foot | style="text-align: center" | 32 Foot | style="text-align: center" | 16 Foot | style="text-align: center" | 8 Foot | style="text-align: center" | 4 Foot | style="text-align: center" | 2 Foot | style="text-align: center" | 1 Foot | style="text-align: center" | 3 Line | style="text-align: center" | 4 Line | style="text-align: center" | 5 Line | style="text-align: center" | 6 Line | |- ! style="text-align: right" | Name | style="text-align: center"