《晋书·志·第六章》
每一宫均有七声，各为调首，交错使用，所以有二十一变。
二十一变也。宫有七声，错综用之，故二十一变也。
Each scale has seven notes, each as the starting note, used interchangeably, so there are twenty-one variations.
<s>[INST] Identify the key themes in this Classical Chinese text and explain them in English: 二十一变也。宫有七声，错综用之，故二十一变也。 [/INST] Each scale has seven notes, each as the starting note, used interchangeably, so there are twenty-one variations.</s>
Identify the key themes in this Classical Chinese text and explain them in English: 二十一变也。宫有七声，错综用之，故二十一变也。
user
Each scale has seven notes, each as the starting note, used interchangeably, so there are twenty-one variations.
assistant