Reference for Niddah 108:23

דאמר קרא (ויקרא טו, לג) והדוה בנדתה מדוה כמותה מה היא מטמאה אף מדוה מטמאה

If a woman is alternately unclean for nine days and clean for nine days she may have marital intercourse on eight days out of every eighteen days.<span class="x" onmousemove="('comment',' In the first group of nine days she is a menstruant during the first seven days and a minor zabah on the last two days; and in the second group of nine days she allows the first day to pass (as prescribed for a minor zabah) while in the remaining eight days, being fully clean, she is permitted marital intercourse. The same process is repeated in every cycle of eighteen day. ');">²⁰</span>

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