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Xenharmonic music is music whose tuning systems do not conform to or closely approximate the common 12-tone equal temperament. The term xenharmonic was coined by Ivor Darreg, from xenia (Greek ξενία), "hospitable," and xenos (Greek ξένος) "foreign." He stated: "This writer has proposed the term xenharmonic for music, melodies, scales, harmonies, instruments, and tuning-systems which do not sound like the 12-tone-equal temperament....Xenharmonics is intended to include just intonation and such temperaments as the 5-,7-, and 11-tone, along with the higher-numbered really-microtonal systems as far as one wishes to go."
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John Chalmers, author of "Divisions of the Tetrachord", writes: "The converse of this definition is that music which can be performed in 12-tone equal temperament without significant loss of its identity is not truly microtonal." Thus xenharmonic music may be distinguished from the more common twelve-tone equal temperament, as well as some use of just intonation and equal temperaments, by the use of unfamiliar intervals, harmonies, and timbres.
What counts as xenharmonic here however is subjective. As an example, Edward Foote in his program notes for his "6 degrees of tonality" CD, refers to this individual difference in response to the more radical tunings he uses, such as Kinberger and DeMorgan, from "shocking", to "Too subtle to immediately notice", saying "Temperaments are new territory for 20th century ears. The first-time listener may find it shocking to hear the harmony change "color" during modulations or too subtle to immediately notice"
Xenharmonic music with recognizable diatonic tunings
Music also can share much of the familiar territory of twelve tone music and yet have xenharmonic features. As examples, Easely Blackwood, author of "The Structure of Recognizable Diatonic Tunings", wrote many etudes in ETs from 12 equal to 24 equal which bring out both many connections and resemblances to twelve tone music as well as bringing out various xenharmonic characteristics of the tunings. See his Twelve Microtonal Etudes for Electronic Music Media.
In his program notes for his Fanfare in 19-et, he writes: "...Triads are smooth, but the scale sounds slightly out of tune because the leading tone seems low with respect to the tonic. Diatonic behavior is virtually identical to that of 12-note tuning, but chromatic behavior is very different. For example, a perfect fourth is divisible into two equal parts, while an augmented sixth and a diminished seventh sound identical. ... The development modulates entirely around the circle of nineteen fifths. "
For a perhaps more radical example, for his sixteen notes Andantino, he writes: "This tuning is best thought of as a combination of four intertwined diminished seventh chords. Since 12-note tuning can be regarded as a combination of three diminished seventh chords, it is plain that the two tunings have elements in common. The most obvious difference in the way the two tunings sound and work is that triads in 16-note tuning, although recognizable, are too discordant to serve as the final harmony in cadences. Keys can still be established by successions of altered subdominant and dominant harmonies, however, and the Etude is based mainly upon this property. The fundamental consonant harmony employed is a minor triad with an added minor seventh."'
Darreg explains: "I devised the term 'xenharmonic' to refer to everything that does not sound like 12-tone equal temperament."
Tunings and instruments
Any scale or tuning other than 12-tone equal temperament can be used to create xenharmonic music. This includes other equal divisions of the octave and scales based on extended just intonation.
Tunings derived from the partials or overtones of physical objects with an inharmonic spectrum or overtone series such as rods, prongs, plates, discs, spheroids and rocks are sometimes used as the basis of xenharmonic exploration. William Sethares is a pioneer in this area. William Colvig, who worked with the composer Lou Harrison created the tubulong, a set of xenharmonic tubes.
The Non-Pythagorean scale utilized by Robert Schneider of The Apples in Stereo, based on a sequence of logarithms, may also be considered xenharmonic.
Electronic music composed with arbitrarily chosen xenharmonic scales was explored on the album "Radionics Radio: An Album of Musical Radionic Thought Frequencies" by British composer Daniel Wilson, who composed his pieces with frequency-runs submitted by users of a custom-built web application replicating radionics-based electronic soundmaking equipment used by Oxford's De La Warr Laboratories in the late 1940s.
Composers
Annie Gosfield's purposefully "out of tune" sampler based music uses non systematic tunings that may be considered xenharmonic. Other composers of xenharmonic music include Elodie Lauten, Wendy Carlos, Ivor Darreg, Paul Erlich and many others.