The legal ruling when a husband conditions divorce on the wife completing half of her menstrual period ('nisf haydah')?
Chapter on Explicit Divorce and Others
Al-Mughni
Book of Divorce
Primary text
If a husband states, "If you complete half a menstrual period, you are divorced," the divorce takes effect upon the completion of half the usual duration of her menses. The ruling must affirm the occurrence of divorce when half her customary period has passed because legal stipulations are tied to the established custom (al-'ādah), and thus the divorce becomes bound to it. This opinion is considered the soundest because menstruation has a defined duration, the minimum being a day and a night, and it possesses a conceptual half-measure. Ignorance regarding the exact measure does not prevent its existence or the attachment of the ruling to it, similar to pregnancy.
Supporting text
A second view suggests that the divorce is not decreed until seven and a half days have passed, as the completion of half a menstrual cycle is only certain through that duration unless she achieves purity sooner, at which point the divorce occurring during that half-period is established. Another opinion states that the stipulation of 'half a menstrual period' is voided, and the divorce remains suspended upon the actual occurrence of menstruation.