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The eleven-string alto guitar or altgitarr is an extended-range classical guitar developed by Swedish luthier Georg Bolin in the 1960s.
Original Bolin instruments are now rare and valuable. The Bolin alto guitar most often has eleven strings, but a thirteen-string version also exists.
They are most often used for performance of lute music, particularly Bach.
In the United States, luthier Walter Stanul makes performance instruments ranging from 11 to 13-strings called the Archguitar.