Xenharmonic music is that which uses a tuning system which neither conforms to nor closely approximates 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 it as being "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."
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.
Theorists other than Chalmers consider xenharmonic and non-xenharmonic to be subjective. As an example, Edward Foote, in his program notes for his "6 degrees of tonality" CD, refers to the differences in his response to the more radical tunings he uses, such as Kirnberger 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.