《宋书·志·卷十一》
凡是用弦乐器伴唱时给乐器调弦的清浊标准不用笛子的尺寸来称呼，就不能够知道。 
凡弦歌调张清浊之制，不依笛尺寸名之，则不可知也。 
When accompanying singing with stringed instruments, the standard for tuning the instruments to pure or impure timbre cannot be called by the size of the flute, so it cannot be known.