Ottava
To be played an octave higher (8va) or lower (8vb).
What it means
Ottava (Italian for "octave") is an instruction to play a passage an octave higher or lower than written. The abbreviation 8va stands above the notes and means "an octave higher", 8vb stands below and means "an octave lower". A dashed line shows how far the instruction applies.
In the notation
The sign 8va with a dashed line stands above the passage concerned and ends with a small hook or with the word loco, which indicates the return to the normal register. For two octaves one uses 15ma and 15mb. An addition such as coll'ottava asks for the passage to be doubled at the octave as well.
Musical use
The ottava avoids many ledger lines above or below the staff, which would make reading harder. It is used above all in piano, organ and orchestral music when passages lie very high or very low. The written text stays clearer while the sound is the same.
The ottava should be distinguished from octave doubling as a sound effect and from loco, which cancels its effect.
Performance practice
When you read an 8va sign, think the note in its written position and then play it an octave higher (or lower with 8vb). Watch for the end of the dashed line and for loco so that you continue in the right register.