[Harry Partch playing his first Adapted Guitar no.1 c.1941. Photograph courtesy of the Harry Partch Archives, University of Illinois at Urbana-Champaign]
Partch's first Adapted Guitar no.1 was a 1927 Martin O-18K parlor guitar that he purchased in New York in 1934. Shortly afterwards – with the aid of a grant from the Carnegie Foundation – Partch departed the United States for London to study the history of intonation at the British Museum. He gradually developed the fretting on this guitar until about 1942, adding frets as new pieces were written and new pitches were required. He replaced the usual low, wire-style frets with high stainless-steel ones fitted into slots in a brass plate, which was screwed onto the neck. The six strings were tuned in three pairs of octaves, separated by the interval of a just major third around Partch's fundamental tone 1/1 [G], producing a justly tuned augmented chord: Eb – G – B.
Partch wrote: “the instrument is played more like a mandolin than a guitar, but its low range of pitch and 2/1 [octave] pairs contribute to a result that is unlike either." He used this guitar to perform the first versions of Barstow , U.S. Highball , December, 1942  and Letter from Hobo Pablo .
In 1945, whilst at the University of Madison in Wisconsin, he replaced this instrument with a fretless electrified archtop guitar, with pinheads and brass rivets delineating an even greater number of microtonal pitches. This three-stringed instrument was also named Adapted Guitar no.1, and tuned exactly like its acoustic predecessor. It was used in a few pieces until about 1956, when it was given to a friend in Northern California and unfortunately lost.
In the same year Partch developed his Adapted Guitar no.2, a ten-string acoustic lap steel guitar played Hawaiian-style, with a Pyrex rod to make gliding pitches possible. This guitar was first used in his Eleven Intrusions , as well as many of the later large-scale ensemble works such as Delusion Of The Fury , and was tuned in various configurations as the works required.
Partch’s final guitar was actually a reworking of his very first guitar. In 1953 he removed the frets of his original Adapted Guitar no.1, painted the fingerboard in bold colours, and also converted it into a lap steel guitar, renaming it Adapted Guitar no.3. All this instrument’s strings were tuned to unisons or octaves of G, and this instrument was even used in his very last work The Dreamer That Remains, composed in 1972.
[My replica of Partch's first Adapted Guitar no.1, as it would have looked circa 1943. Photograph taken in London in 2015]
[Partch playing his Adapted Guitar no.2, possibly in the 1950's. Photograph courtesy of the Harry Partch Archives, University of Illinois at Urbana-Champaign]
[Partch playing his Adapted Guitar no.3, possibly in the 1950's. Photograph courtesy of Elisabeth Smalt]