Purity during the menstrual period is a valid period of purity. Thus, if she sees a day of purity and a day of blood, and it does not exceed the maximum of menstruation, she combines the blood with the blood, and it becomes menstruation, while the purity between them is a period of purity, according to what we have established. There is no difference between the duration of the blood being more than the duration of the purity, [or equal to it], or less than it, such as if she sees two days of blood and one day of purity, or two days of purity and one day of blood, or less or more; all the blood is menstruation if it recurs [and the duration does not exceed] the maximum of menstruation. If the blood is less than a day, such as if she sees half a day of blood and half a day of purity, or an hour and an hour, our companions said: It is like full days; the blood is combined with the blood, and it becomes menstruation, and the purity between them is a period of purity, if the combined amount reaches the minimum of menstruation. If it does not reach that, it is blood of dysfunction (istihadah). There is another opinion regarding it, that the blood does not become menstruation unless it is preceded by a valid, continuous menstruation. All of this is the school of al-Shafi'i. He has an opinion that purity between two bloods is menstruation, which we have mentioned, and we also mentioned a position of ours that whenever purity is less than a day, it is not a period of purity. Therefore, whenever the purity is less than a day, the blood and what is between it are all menstruation. If the blood exceeds the maximum of menstruation, such as there being more than fifteen days between its two ends—for instance, if she sees a day of blood and a day of purity for eighteen days—then she is a woman suffering from istihadah. She cannot be anything but one who has a habit, or one who has a faculty of discrimination (tamyiz), or one who has neither a habit nor discrimination, or one in whom both apply. If she has a habit, such as her menstruation being five days at the beginning of every month, she sits [observes the rules of menstruation] for the first day she sees blood during her habit, and she performs ghusl when it ceases. What follows that is built upon the two narrations regarding purity during the menstrual period: does it prevent what follows it from being menstruation, or not? If we say it prevents it, then her menstruation is the first day only, and what follows it is istihadah. If we say it does not prevent it, then her menstruation is the first, the third, and the fifth days; thus, she obtains three days from her habit, and the rest is