An octave twelve is a type of 12-string guitar fitted with a short-scale neck (15.5 inches) and a small solid body. It is tuned one octave higher than a standard guitar, giving it the tonal range of a mandolin and enabling a guitarist to achieve a mandolin sound without learning mandolin fingering. The effect is similar to that of applying a capo to a standard 12-string guitar at its twelfth fret.^{[1][failed verification]} However, unlike a standard 12-string guitar, the courses of strings are tuned in unison rather than in octaves.^{[citation needed]}

The octave twelve was invented by engineers at Vox, which sold the octave twelve as the mando-guitar from 1964 to 1968. Notable users of the mando-guitar included Brian Jones of the Rolling Stones. Most modern octave twelves are modelled after the distinctive body shape of the Vox mando-guitar. It was also used on the introduction of the Beach Boys' "Wouldn't It Be Nice", from "Pet Sounds".^{[citation needed]}