>>106Two elements define the consonance of an interval. First, the size of the numbers in the ratio. For example, 5:4 (the just major third) will sound more consonant than something like 81:64 (the Pythagorean major third). Second, how close are the notes to the actual interval. On a piano and most modern western instruments that uses equal temperament, octaves (2:1) are pure, perfect fifths are almost pure, but major thirds (5:4) less so, even if we got used to it over the time.
3:2, the perfect fifth, was the interval that Pythagoras used to build his Pythagorean scale, which served as the base for the western tuning system. It's using the Pythagorean tuning the modes mentioned at
>>96 were made.
5:4, the major third, and 6:5, minor third, were "discovered" in the Renaissance, and a tuning called meantone was invented to have these intervals as pure as possible to go with the music written at that time. The problem with meantone was that you could play in only a certain number of keys without the intervals sounding badly out of tune.
Meantone was eventually replaced by what we now call Well-Temperament (as in Bach's "The Well-Tempered Clavier") and later by Equal Temperament so that the music could be played in any key without sounding out of tune.
The notes in 12 tone Equal Temprament can be mathematically calculated by the formula 440Hz*2^(x/12) where x is the number of notes higher or lower than middle A. It also happens sometimes that the 440Hz is changed a bit for whatever reason.
This information probably has little value to someone that's trying to learn modern music theory, but I find it interesting anyway.