Eight-string guitar

Various non-standard guitars were made in the 19th century, including eight-string guitars played by Italians Giulio Regondi and Luigi Legnani.

Eight-string electric guitars gained popularity among metal bands, largely inspired by Swedish progressive metal band Meshuggah (formed in 1987). Contemporary use outside of metal has picked up in the last decade, and owes much to Animals as Leaders and their stylistic eclecticism.

Designs

Semi-acoustic guitar (hollow-body guitar)

Seeking a guitar tuning that would facilitate jazz improvisation, Ralph Patt invented major-thirds tuning in 1963. Patt's tuning is a regular tuning, in the sense that all of the intervals between its successive open strings are major thirds; in contrast, standard guitar tuning has one major-third amid four fourths.

Seven-string guitars are needed for major-thirds tuning to have the E-e' range of the standard tuning. Having an eight-string instrument allowed Patt's guitar to have G♯ (equivalently A♭) as its open note. Patt purchased six-string archtop hollow-body guitars that were then modified by luthiers to have wider necks, wider pickups, and eight strings. Patt's Gibson ES-150 was modified by Vincent "Jimmy" DiSerio c. 1965. Luthier Saul Koll modified a sequence of guitars: a 1938 Gibson Cromwell, a Sears Silvertone, a c. 1922 Mango archtop, a 1951 Gibson L-50, and a 1932 Epiphone Broadway; for Koll's modifications, custom pick-ups accommodated Patt's wide necks and high G♯ (equivalently A♭); custom pick-ups were manufactured by Seymour Duncan and by Bill Lawrence. Roy Connors, former member of the 1960s folk singing group, The Highwaymen, reconfigured a Martin O-28 six-string guitar to an eight-string of his own design and received a U.S. Patent on it (#3269247).

Solid-body

Solid-body eight string guitars are also used by many bands today. The construction of a solid-body eight-string guitar is comparable to that of seven- and six-string variants. The standard tuning (from low to high) is F♯, B, E, A, D, G, B, E. Many prefer to tune the F♯ to a low E (E1), the same note as the lowest string on a four-string electric bass in standard tuning, and providing the guitar with a fuller sound by having three different E strings. This tuning is equivalent to tuning a six-string guitar to Drop D tuning.

Like the seven-string, the first mass-produced eight-string guitar was made by Ibanez guitars in Japan; the RG2228.

Scale length

The main design issue faced with an eight-string guitar is tuning stability with the lower strings. This is due to the neck being constructed too short, bridge problems such as improper intonation, uneven spacing for floating bridges, or the use of wrong string gauges. Other problems associated with tuning stability rely on the proper setup of the guitar.

Extended range eight string guitars sometimes will have a multi-scale design where the bass strings will be longer than the treble strings (fanned fret design). This helps with proper intonation of the lower strings, improves string tension balance across the strings, improves harmonic overtones, overtone series, and improves inharmonicity. (See also inharmonicity in pianos). The bass strings on an 8 string typically require the saddle to be pulled back a bit more than the other strings to properly set the intonation. Some bridge designs accommodate this by offsetting back the 7th and 8th strings or providing a bit extra room for adjustment. Longer scale lengths require less offset for proper intonation.