Originally, every musical scale in the world has irregular spaces between its steps. This is because looking for the consonances between notes naturally produce an irregular distribution. But if we want to change the tonality in the middle of the music that we are composing, it won’t work with such scale because most of the steps will not mach anymore as illustrated in the picture below (some consonant intervals will become very dissonant). This is why european instruments couldn’t play polyphony before the XIV century while vocal music had four voices since the XII century, and this is why Indian music, based on very rich irregular scales, is, by principe, “monotonal”, with always this never-ending drone played behind by the tampura, and this also why 99% of the instruments worldwide are now using the european “Equal Temperament” which, by equalizing all the steps in the musical scale, permit to change the tonality freely anytime… with the downsides that almost none of the intervals are truly consonants and that the playable consonances are by construction rather limited in quantity. Of course fretless instruments, and voices, can still improvise around it with more rich harmonic content, but the structural basic of any music in the world is – until now – a fixed scale. Except, to some extent, in some very rare and hard to learn polyphonic singing traditions like for example Bulgarians Voices, the possibility of combining all kind of consonances with the ability to change tonality anytime, is a goal that haven’t been reached yet.