《明史·志·卷三十七》
因为阳数起于子而终结于少阴的申，阴数起于午而终结于少阳的寅。
盖阳数起子而终于少阴之申，阴数起午而终于少阳之寅。
This is because yang numbers begin with Zi and end with the lesser-yin Shen, and yin  numbers begin with Wu and end with the lesser-yang Yin.