related: chords
Consonant intervals have simple frequency ratios.
Dissonant intervals do not. E.g. the tritone (3 whole tones), or aug4/dim5, has a ratio of 5:7.
In a way, harmony illustrates our preference for simplicity.
See also: harmonic series
(E.g. to go up a fifth, multiply the note’s frequency by 3/2. )