Reference for Niddah 108:24

אשכחן לח יבש מנלן

If she is alternately unclean for ten days and clean for ten days, the days in which she is permitted marital intercourse are the same in number as the days of her <i>zibah</i>.<span class="x" onmousemove="('comment',' During the first ten days she is a menstruant for seven days and a zabah during the last three days, while during the second group of ten days she counts the prescribed seven days and has three days left in which she is clean and permitted marital intercourse. The three latter days are thus equal in number to the three days of her zibah. ');">²¹</span>

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