It's a different take on trying to set up the pitches on a piano/organ/synth keyboard so you can shift between keys and everything still sounds tuneful. Ideall

It's a different take on trying to set up the pitches on a piano/organ/synth keyboard so you can shift between keys and everything still sounds tuneful. Ideally you want exact numeric ratios between the various note combinations. (E.g. the frequency of a G should be 1.5 times the frequency of a C.) If you start with a C and set up perfect ratios, you get a smooth sound as long as you play in the key of C. But if you change key and move pitch patterns up and down the keyboard - transposition - the pitches will be off. Sometimes by a lot. So you have these different systems that try to approximate a solution in different ways. Modern Western music uses Equal Temperament, where the frequency ratio between notes is the same all the way up the keyboard (exactly the 12th root of 2.) This is super-simple and gives close-enough ratios that aren't quite exact but are very usable. The cost is a hint of sourness in some note combinations, but mostly we just hear those as movement within the sound. The older alternatives to Equal (mean tone, Werckmeister, Kirnberger, and others) smooth out the ratios more selectively. So some note combinations are less sour, others are more sour. They sound quite spicy if you're not used to them - somewhat out of tune (ironic...) but most people are so used to Equal Temperament anything else sounds exotic. So something like this is of historic interest. These tunings are sometimes used for authentic performances of very old ('early') music, and some old church organs were built with them. It's very rare to find them being used for effect in modern songs. (Does happen, but very unusual.) The best solution is dynamic tuning which takes into account the surrounding notes and shifts pitches a little up and down for the smoothest effect. Choirs and orchestras tend to do this anyway, but making it happen in software is a much harder problem.