The Cycle of 5ths. GX1, Leitz Summicron 50mm DR

Modern pianos are tuned in what is called Equal Temperament. A short explanation of this is to say that all of the major and minor keys are equally *out* of tune by the same amount. It's a convenient compromise and it wasn't always this way; there are many different temperaments which try to resolve the conundrum of Pythagorean Comma. The convenience is that one can change keys, migrating through all the degrees of tonic and dominant, and eventually arrive back exactly at the starting point. This is the Cycle (or Circle) of Fifths.