סימן קנד סעיף ב
אֲבָל רֹב הַנָּשִּׁים, דַּרְכָּן לִקְבֹּעַ וֶסְתָּן בְּהַפְלָגוֹת שָׁווֹת, דְּהַיְנוּ שֶׁהִיא מַפְלֶגֶת וּמְחַלֶּקֶת בֵּין רְאִיָה לִרְאִיָה, יָמִים שָׁוִים בְּמִסְפָר. כְּגוֹן שֶׁהִיא רוֹאָה פַּעַם אַחַת וּמַפְסֶקֶת חֲמִשָּׁה וְעֶשְׂרִים יוֹם אוֹ שְׁלֹשִׁים יוֹם אוֹ שְׁנַיִם וּשְׁלשִׁים יוֹם וְכַדוֹמֶה, וְחוֹזֶרֶת וְרוֹאָה. אִם עָשְׂתָה שָׁלֹשׁ הַפְלָגוֹת שָׁוֹות וּרְצוּפוֹת, הֲוֵי לָהּ וֶסֶת קָבוּעַ. וְזֶה נִקְרָא וֶסֶת הַפְלָגוֹת. וּלְאַחַר שֶׁתַּפְלִיג שׁוּב יָמִים בְּמִסְפַּר אֵלּוּ, אֲזַי הַיוֹם הַבָּא הוּא יוֹם וֶסֶת קָבוּעַ שֶׁלָּהּ. וְכֵיוָן שֶׁכָּל וֶסֶת אֵינוֹ נִקְבָּע בְּפָחוֹת מִשָּׁלשׁ פְּעָמִים, וְהַפְלָגָה אֵינָהּ נִכֶּרֶת אֶלָּא בִּשְׁתֵּי רְאִיּוֹת, לָכֵן לְוֶסֶת הַפְלָגוֹת צְרִיכוֹת אַרְבַּע רְאִיוֹת, כְּגוֹן שֶׁרָאֲתָה הַיּוֹם, וְהִפְלִיגָה חֲמִשָּׁה וְעֶשְׂרִים יוֹם וְרָאֲתָה, וְשׁוּב הִפְלִיגָה חֲמִשָּׁה וְעֶשְׂרִים יוֹם וְרָאֲתָה, וְשׁוּב הִפְלִיגָה חֲמִשָּׁה וְעֶשְׂרִים יוֹם וְרָאֲתָה הֲרֵי רָאֲתָה אַרְבַּע פְּעָמִים, אֲשֶׁר בֵּינֵיהֶן שָׁלֹשׁ הַפְלָגוֹת שָׁוֹות, וְקָבְעָה וָסֶת.
Most women, however, are accustomed to establish a mensal pattern of "equal intervals", which is a separation from one perception to another of an equal number of intervening days. For instance, if she perceives a flow and twenty-five days elapse, or thirty days or thirty-two days or the like, and following she again perceives a flow. If she had these perceptions on three consecutive equal1If this sequence had been interrupted by a flow after a shorter interval elapsed, this does not upset the establishment of the longer interval. For example, if she perceived after twenty days on two consecutive occasions, and then perceived after a fifteen day interval, and then again after a twenty day interval, she has thus established a twenty day cycle. But if, instead of the fifteenth, she had seen on the twenty-fifth, she has not established her menses. Chazon Ish 85:34. intervals, this is her established date of the menses, and is referred to as the menses of "equal intervals." And after another interruption of that number of days, the following day is her established date of the menses. Since the menses cannot be established by less than three times, and an interval cannot be recognized by less than two perceptions; therefore, establishment of the menses of equal intervals requires four perceptions. For instance, if she perceived a flow today, followed by an interval of twenty-five days, and she perceived, followed again by an interval of twenty-five days, and she perceived, followed again by an interval of twenty-five days, and she perceived, she has had four perceptions, between which there were three equal intervals, and has thereby established her menses.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)