Xenharmonic musical tuning systems are those that don't conform to or closely approximate the common 12-tone equal temperament. The term was coined by Ivor Darreg, from xenia (Greek ξενία), "hospitable," and xenos (Greek ξένος) "foreign." He famously 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."
Xenharmonic tunings include those such as 5- and 7-tone equal temperament, which are perhaps excluded under "microtonal" rubric, since their intervals are larger than those of 12-ET. The term "microtonal" may have also been seen as too restrictive in that it was strongly associated with the quartertone movement, and with composers like Julián Carrillo, who only worked in equal temperaments that were multiples of 12. Darreg was among the first to argue that any equal temperament could be a valid source of musical materials.
Xenharmonic tunings 
Tunings derived from the overtones of physical objects with an inharmonic 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.
Xenharmonic 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.
Further reading 
See also 
- Microtonality - Web in spanish
- Microtonal music on CD
- Homepage for William Sethares
- xenharmonic wiki on wikispaces
- Xenharmonic Alliance
- Barbieri, Patrizio. Enharmonic instruments and music, 1470-1900. (2008) Latina, Il Levante Libreria Editrice