Seven-string guitar string is augmented with a high octave string. Many of McGuinn's notable guitar solos utilize the G string of the twelve-string guitar to perform the main melody, and therefore the Martin seven-string guitar was designed to achieve this extended range playing without the need for doubling all six of the guitar's strings. In 2010, Inox Guitars has created a mix of the Brazilian Viola caipira and the Russian semistrunka (семиструнка). This instrument has 2 bass strings (as the 6th and 7th strings of the semistrunka) and five treble courses (as the Viola caipira) and it is used in open tuning
Fret generally perpendicular to the instrument's neck centerline and parallel to each other, on a "fanned" fretboard, the frets are angled (spread like a fan) with only one center fret perpendicular to the neck’s centerline. This gives the lower-pitched strings more length and the higher strings shorter length (comparable to a piano or a harp where heavier strings have different lengths). The idea is to give more accurate tuning and deeper bass. Some think that fanned frets might be more ergonomic. Fanned frets first appeared on the 16th century Orpharion, a variant of the cittern, tuned like a lute. John Starrett