Comma

stack twelve perfect fifths. the circle won’t close.

tap up twelve pure fifths round a clock of notes, and hear them miss C

Twelve fifths tuned by pure 3:2 ratios overshoot seven octaves by 23.5 cents -- the Pythagorean comma. Equal temperament closes that gap by flattening each fifth by 2 cents, trading pure intervals for a circle that actually closes.

Concepts: harmony, tuning.

An interactive you play with one thumb, on Wundr.