Some of the harmonics on the series would be quite out-of-tune if played on a regularly tuned, equal-tempered guitar.Tenney obviates this problem by retuning a couple of strings on the guitars responsible for each harmonic—and restricting the musical material for that guitar only to those notes that can be sounded accurately in tune.

The repertoire of American composer James Tenney (1934-2006) is among the most diverse and stimulating in experimental music.
He studied with, amongst others Chou Wen-Chung and John Cage.Tenney’s work deals with perception (For Ann (rising), see Shepard tone), just intonation (Clang, see gestalt), stochastic elements (Music for Player Piano), information theory (Ergodos, see ergodic theory), and with what he called ‘swell’ (Koan: Having Never Written A Note For Percussion for John Bergamo), which is basically arch form.

His earliest works show the influence of Webern, Ruggles and Varèse, whereas his music from 1961-64 was largely computer music, one of the earliest significant body of such work in existence.
Tenney was one of the four performers of the Steve Reich piece Pendulum Music on May 27, 1969 at the Whitney Museum of American Art. The other three were: Michael Snow, Richard Serra and Bruce Nauman. Tenney also performed on Terry Riley‘s album In C.
His works which specifically include the guitar are:
Spectrum 5. flute, oboe, bass clarinet, bassoon, vibraphone et al., harp, piano, viola; (1995); 16′. Written for Champ d’Action. Copyright JT. [T] Spectrum 4. Violin, alto recorder, vibraphone, guitar, piano, bass clarinet, tenor-bass trombone, contrabass; (1995); 16′. Written for the Maarten Altena Ensemble
Sneezles. Soprano, flute or alto recorder, clarinet, vibraphone, violin, guitar, tenor-bass trombone, contrabass; (1995);
‘Water on the Mountain . . . Fire in Heaven‘. 6 electric guitars (1985); 25′. [R: Seth Josel, Long Distance, CRI CD 697
Septet. 6 electric guitars and bass (1981); 5’30”. [R: Mills College Contemporary Ensemble,Tellus cass.

James Tenney’s Septet (1981)

Read more at at The Classical Guitar website

More about James Tenney

(pretty exhaustive further reading and music examples

James Tenney’s Septet (1981)http://youtu.be/m-oN7El5VM8