Halakhah for Niddah 144:20

From the laws of the commandment is that which they, may their memory be blessed, said (Niddah 72b) that there are eleven days between [one] menstruation (niddah) and [another] menstruation - meaning to say, that she becomes a zavah upon them. This is a law of Moshe from Sinai. And the explanation of the matter is like this: That every woman at the time that she [experiences] blood, at the beginning of her [experiencing] it, is called a 'menstruant.' And the law of the matter of the menstruant is like this: That if she [experiences] blood one day, or even seven consecutive days - so long as the blood stops on the seventh day while it is still day - immerses at night; which is the night, the morrow of which is the eighth from the beginning of her experiencing [it]. And after her immersion, she is pure for her husband. But after the seven days of menstruation, if she [experiences] blood within the eleven days after the seven, that blood is called, 'blood of discharge,' and its law is [as follows]: That if she [experience] it for one day - whether she [experiences] it at the beginning of the night or the end of the day - she watches one day corresponding to it and immerses, and she is pure for her husband in the evening. And her immersion is even during the day, from when the sun rises. And this is called a 'small zavah.' And so [too,] if she [experiences] blood two days and stops on the third, her law is likewise to watch one day. And this is sufficient for her - even for two days - and she is purified. And this is also called a 'small zavah.' But if she [experiences] blood during these eleven days [over] three consecutive days, she is called a 'big zavah,' and needs to sit [out] seven clean days. And afterwards, she immerses and is pure for her husband. And after these eleven days, if she [experiences blood], she goes back to the beginning of menstruation; and she has seven days [to be] like the law of the menstruant, as we said above. And so is the matter always - that after [the days of] menstruation, she has eleven days in which she [can] become a zavah. And after those eleven days pass, she never becomes a zavah until the seven days of menstruation have passed.