How is the customary menstrual period established?

Primary text

The customary menstrual period (*adah*) is established by discernment (*tamyeez*). If a woman sees black blood for five days within three months (or two months, according to another narration), and then the blood changes to red and continues, and in the remaining months, she sees unspecified blood (*damm mubahham*), her customary period is established as the duration of the black blood.