read up on the Pythagorean comma and also equal temprament . the chromatic scale is a very neat system for ordering the mathematical complexity to cover just a

read up on the Pythagorean comma and also equal temprament . the chromatic scale is a very neat system for ordering the mathematical complexity to cover just about all the major relating 'fractions' of each note and relate these to each other. its a European thing really which has since been adapted onto many cultures with traditionally different tuning systems. the main area of the world where other systems remain is the Indian subcontinent which has more complex and 'microtonal' scales. in the 20th century Harry Partch was a leading light on microtonal music - this basically requires the creating of his own instruments. many of his most important work was based on trying to understand ancient Greek scales and tuning. personally I believe that 'microtonal' music stands to make a big comeback as computers (on which more and more music is made nowadays) will make this easier than traditional orchestras.