If an instrument has a range that does not fit on the staff well when using one of the common clefs its music may be written on treble or bass clef, with the pitches sounding an octave higher or an octave lower than written. This avoids the use of excessive ledger lines. These instruments are said to "transpose at the octave" — they are not playing in a different key from concert pitch instruments, but sound one or more octaves higher or lower than written. Music for the double bass sounds an octave lower than written, music for the piccolo sounds an octave higher.