equal temperament
in which "semitones are slightly adjusted to make all the intervals of the octave equal"
J. S. Bach's "Well-Tempered Clavier" explores and publicizes the possibilities of equal temperament, and it "has dominated Western music since the beginning of the eighteenth century. Fixed-pitch modern instruments such as the piano and organ are tuned to equal temperament"
but Terry Riley and La Monte Young move away from the twelve tones of equal temperament
Harry Partch as well (see his Genesis of a Music)
retuned keyboards are "returned to an accord with the physical laws of nature"
Riley describes them as "resonant intervals, the intervals that are really in tune"
"In the coming years, the frontier will be tuning"