 Starting in the 18th-century treble clef has been used for transposing instruments that sound an octave lower, such as the guitar; it has also been used for the tenor voice.To avoid ambiguity, modified clefs are sometimes used, especially in the context of choral writing; of those shown, the C clef on the third space, easily confused with the tenor clef, is the rarest.This is most often found in tenor parts in SATB settings, in which a treble clef is written with the numeral 8 below it, indicating that the pitches sound an octave below the written value.