وَالْمُطَلَّقَاتُ يَتَرَبَّصْنَ بِأَنْفُسِهِنَّ ثَلَاثَةَ قُرُوءٍ
Divorced women are to wait for three qurūʾ. (Qur'an 2:228)
Hanafi jurists understand qurūʾ here as menstrual periods, while Shafiʿi jurists understand them as periods of purity. Neither position changes the specified number three; they differ on which units are counted.
Menstruation reading: the waiting period is reckoned by three menstrual periods.
Purity reading: it is reckoned by three periods of purity.
This difference affects legal conclusions about the final stage of ʿiddah, revocation, remarriage and some continuing rights.