mr. beardsley can likely explain this to shreds. but here're the parts that strike me as most signficant: 1. a scale in just intonation, as david said, derives its notes from the harmonic overtones series. hence the argument by harry partch and others that it is "purer" than equal temperment because JI uses notes as they occur in nature, while ET is a manmade scale. 2. again, to say it just a bit differently than david, a scale in just intonation generally finds a different sized interval between each successive note. that's why modulation is more difficult and that's the problem that was addressed by equal temperment -- or "well tempered" as they described that famous klavier. in equal temperment, the interval of a half step between each successive note is exactly the same. martin