The twelve string guitar is an acoustic or electric guitar with 12 strings, which produces a richer, more ringing tone than a standard six string guitar.
The strings are placed in courses of two strings each that are usually played together. The two strings in each bass course are normally tuned an octave apart, while each pair of strings in the treble courses is tuned in unison. The tuning of the second string in the third course (G) varies: some players use a unison string which is less prone to breakage, others prefer the distinctive high-pitched, bell-like quality an octave string makes in this position.
Some players, either in search of distinctive tone or for ease of playing, will remove some of the doubled strings. For example, removing the higher octave from the three bass courses simplifies playing running bass lines, but keeps the extra treble strings for the full strums.
The tension placed on the instrument by the strings is great, and because of this, 12 string guitars have a reputation for warping after a few years of use. Some 12-string guitars have nontraditional structural supports to prevent or postpone such a fate, at the expense of appearance and tone. Until recently, twelve-string guitars were nearly universally tuned lower than the traditional EADGBE, to reduce the stresses on the instrument. Leadbelly famously used a low C-tuning, as did Beau on his Dandelion recordings.
Many performers who play the 12-string guitar use an ordinary six-string guitar as their primary instrument, switching to the twelve-string guitar for certain songs that seem to call for a brighter sound..