Exploring Equal Temperment
The two sets of horizontal lines mark out one traditionally accepted set of just intervals for the twelve notes of the (western) chromatic scale. The dashed lines pick out the notes of the major scale.
The blue and red sets of dots represent the equal division of the octave into however many pieces the blue and red sliders are respectively set to. One can see nice approximations of the entire chromatic scale with 12 notes. This is the system of equal temperment that underlies most modern western music. Exploring with the sliders, other systems of equal temperment (called TETs) can be found. 41-TET is particularly well known, but others exist as well. 65-TET for example gives a very good approximation of each of the pure ratios listed on the left.