That there are eleven days between one menstrual period and another is a law from Moshe at Sinai. Its explanation is that the law transmitted to Moshe at Sinai is like this: From the beginning of any woman's seeing menstrual blood, her law is to count for all of her days, seven days of menstruation (<i>niddah</i>) from the beginning of her seeing. And after these seven days of menstruation, [she counts] eleven days, and they are called the days of flow (<i>zivah</i>). And after these eleven days that are days of flow, she goes back to the seven days of menstruation. And after them eleven days of flow, and continuing like this. And so is it fitting to count in this way all of her days. And these seven days of menstruation are whether she saw one day or even one hour, or even if she saw all of the entire seven days of menstruation. In the evening, she immerses [in the <i>mikveh</i> and is permitted to her husband, and does not require clean [days]. As it is stated (Leviticus 15:19), "seven days shall she be in her menstruation" - she shall be in her menstruation all seven. But on these eleven days between [one] menstruation and [another] menstruation, any day that she sees blood, she observes one day corresponding to that day, and that is [the law of] a small flow. And if she did not see on the second day, she immerses and becomes pure. But if she saw also on the second day, she observes a day corresponding to the two days that she saw. And if she did not see on the third day, she immerses and becomes pure. But if she saw also on the third day that she is observing, she saw three consecutive days. [Hence] she is [in the category of] a large flow. And [so] she needs to sit seven complete (twenty-four hour) days after the blood has ceased from her. And she does not count the day that the blood ceases for the count of clean days. And this is the law in the days of flow: Any time she saw one day, she counts [another] day corresponding to it; and so [too,] two days, she counts one day for them. [But if] she saw three consecutive days, she observes seven days.