The twelve-string guitar is an acoustic or electric guitar with 12 strings in 6 courses, which produces a richer, more ringing tone than a standard six-string guitar. Essentially, it is a type of guitar with a natural chorus effect due to the subtle differences in the frequencies produced by each of the two strings on each course.
The tension placed on the instrument by the strings is high, and because of this, 12 string guitars have a reputation for warping after a few years of use. Some twelve-string guitars have non-traditional structural supports to prevent or postpone such a fate, at the expense of appearance and tone. Until recently,[when?] twelve-string guitars were nearly universally tuned lower than the traditional EADGBE, to reduce the stresses on the instrument. Leadbelly may have used a low C-tuning.
Because it is substantially more difficult to pluck individual strings on the twelve-string guitar, and almost impossible to bend notes tunefully, the instrument is rarely used for lead musical parts. 12-string guitar is however primarily suited to a rhythm or accompaniment role and is often used in folk songs and some popular music. Some hard rock and progressive rock musicians use double-necked guitars, which have both six-string and twelve-string components, allowing the guitarist easy transition between different sounds.
The double ranks of strings of the 12-string guitar produce a chorus effect, because the individual string sounds with roughly the same timbre and nearly (but never exactly) the same pitch converge and are perceived as one. When the effect is produced successfully, none of the constituent sounds is perceived as being out of tune because of the minute difference in pitch. The interference between the slightly different frequencies produces a phenomenon known as a beat that manifests itself as the periodic rise and fall of intensity is considered pleasing to the ear. The effect is more apparent when listening to notes that sustain for longer periods of time.