A piano usually consists of 88 keys, including 52 white keys and 36 black keys. Fifty-two white keys on the piano repeatedly use seven basic tone names.
Therefore, many sounds with the same name are produced in the sound series. In order to distinguish the sounds with the same name but different pitches, we divide the sound series into many "groups".
each group contains seven white keys and five black keys keys, seven white keys express seven basic tone names, and five black keys keys fill the semitones between the white keys.
The so-called "law of twelve averages" is to make the interval distance between every two adjacent keys in each group equal.