Notation
Octaves are identified with various naming systems. Among the most common are the Scientific, Helmholtz, Organ Pipe, Midi, and Midi Note systems.
In writing a specific octave is often indicated through the addition of a number after the note letter name. Thus 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 above is "C5", in a system known as scientific pitch notation.
The notation 8va is sometimes seen in sheet music, meaning "play this an octave higher than written" (all' ottava: "at the octave"). 8va stands for ottava, the Italian word for octave (also meaning "eighth"). Sometimes 8va is used to tell the musician to play a passage an octave lower, though the similar notation 8vb (ottava bassa) is more common. Similarly, 15ma (quindicesima) means "play two octaves higher than written" and 15mb (quindicesima bassa) means "play two octaves lower than written." Col 8 or c. 8va stands for coll'ottava and means "play the notes in the passage together with the notes in the notated octaves". Any of these directions can be cancelled with the word loco, but often a dashed line or bracket indicates the extent of the music affected.
For music-theoretical purposes (not on sheet music), octave can be abbreviated as P8 (which is an abbreviation for Perfect Eighth, the interval between 12 semitones or an octave).
Read more about this topic: Octave