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The seven-string guitar adds one additional string to the more common six-string guitar, commonly used to extend the bass range (usually a low B) or also to extend the treble range.
The additional string is added in one of two different ways: by increasing the width of the fingerboard such that the additional string may be fretted by the left hand; or, by leaving the fingerboard unchanged and adding a "floating" bass string. In the latter case, the extra bass string lies next to the existing bass strings, but free of the fingerboard in similar fashion as the archlute and theorbo. Such unfrettable bass strings were historically known as diapasons or bourdons.
Some types of seven-string guitars are specific to certain cultures such as the Russian and Brazilian guitars.

Source: Wikipedia