How is the definite duration of menstruation established when the known duration exceeds half of the assumed period?
Chapter on Menstruation
Al-Mughni
Book of Purification
Primary text
When the known duration is greater than half the time, the excess beyond half is doubled and established as definite menstruation. For example, if she is accustomed to six days in the first ten, the sixth day is doubled, making the fifth and sixth days definite menstruation. The remaining four days are observed through estimation or counted from the start of the first ten days. If counted from the start, two days are definite menstruation, four are suspect menstruation, and the remaining four days of the ten are suspect purity.
Supporting text
If she observes the remaining four days at the end of the known time frame, they are suspect menstruation, and the initial four days are suspect purity. If she observes seven days in the first ten, two days exceed half the assumed duration and are doubled, making days four through seven definite menstruation. The remaining three days are observed from the beginning of the first ten days or by estimation, making them suspect menstruation, and the remaining three days of the month suspect purity. Suspect menstruation carries the same ruling as definite menstruation regarding abandonment of acts of worship.