The הפסק טהרה to stop in purity (תפסוק בטהרה) close to בין השמשות, the מוך דחוק of the single day, the חלוק בדוק and the לבנים, counting the שבעה נקיים from the next day, the two בדיקות per day and the cloth of the edim, the special cases (blind, deaf-mute, ill), the continuity of the seven days and the סתירה, and the פולטת שכבת זרע (counting from the 5th day) — a central and very practical subject, to be learned from the Rav and the moret kalla (Shulchan Aruch, Yoreh De'ah 196 — 13 se'ifim)
צְרִיכָה שֶׁתִּפְסֹק בְּטָהֳרָה, וְזֶהוּ הַנִּקְרָא הֶפְסֵק טָהֳרָה. וְכֵיצַד עוֹשָׂה? בּוֹדֶקֶת עַצְמָהּ בַּיּוֹם שֶׁפָּסְקָה בּוֹ, סָמוּךְ לְבֵין הַשְּׁמָשׁוֹת, וְלוֹבֶשֶׁת חָלוּק בָּדוּק; וּלְמָחֳרָת מַתְחֶלֶת לִסְפֹּר שִׁבְעָה נְקִיִּים, וּבוֹדֶקֶת בָּהֶם שַׁחֲרִית וְסָמוּךְ לְבֵין הַשְּׁמָשׁוֹת.
Before counting, the woman must stop in purity (תפסוק בטהרה) — this is the הפסק טהרה. How does she do it? She checks herself on the day she ceased, close to בין השמשות, puts on a חלוק בדוק ; from the next day she begins counting the שבעה נקיים, and during them she checks in the morning (שחרית) and close to בין השמשות.
Shulchan Aruch, Yoreh De'ah 196:1-4 (summary)LEVEL 01
The vocalized Hebrew text of the 13 se'ifim with a fluent English translation: the הפסק טהרה close to בין השמשות, the מוך דחוק of the single day, the חלוק בדוק and the לבנים, counting the שבעה נקיים from the next day, the two בדיקות per day, the cloth of the edim, the special cases and the פולטת — explained with practical cases, always referring to the Rav and the moret kalla.
📖 StudyLEVEL 02
In-depth pilpul: the nature of the הפסק טהרה (why stopping בטהרה is indispensable before counting, §1,5) and the סמוך לבין השמשות לכתחילה; the מוך דחוק of the single day (§2) — the מחלוקת Rashba/Ramban and the Rama's בדיעבד; the שיעור הבדיקה and the כלי (בגד פשתן ישן / צמר גפן, עד מקום שהשמש דש in the crevices, אדמומית in daylight); the רצופים / סתירה (why one sighting voids all, even on the eve of the 7th day) and the Rama's קולות on the כתם of the first three days; and the גזרת פולטת שכבת זרע, the six עונות and the מניין מיום ה' (link to 193). חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (the hefsek and the מוך דחוק, the חלוק and the לבנים, the בדיקות of the seven days, the cloth and manner of the edim, the סתירה and the פולטת), golden rules, classic pitfalls (סמוך לבין השמשות, עד מקום שהשמש דש, רצופים, יום ה') and memorization of the 13 se'ifim grouped into 6 families — always under the guidance of the Rav and the moret kalla.
📖 StudyLEVEL 04
The shitah of the Tzemach Tzedek of Lubavitch and the Chabad mesorah on the הפסק טהרה, the בדיקות and the מוך דחוק — a living Chabad domain, presented at the level of principle while referring for conduct to a Chabad Rav —, then the halacha le-ma'aseh according to the Beit Yosef, the Rama, the Shach, the Taz, the Sidrei Tahara, the Chochmat Adam, the Aroch haShulchan and contemporary pesak (Taharat haBayit, Shevet haLevi, Badei haShulchan). Every conclusion refers to the Rav and the moret kalla.
📖 StudyThe הפסק טהרה is the stopping בדיקה by which the woman stops in purity (תפסוק בטהרה) on the day she ceased to see blood: without a hefsek in purity, one never begins counting the שבעה נקיים. The Shulchan Aruch (YD 196) teaches that she checks herself that day, לכתחילה close to בין השמשות (before sunset), after washing, putting on לבנים (white garments), and that from the next day she begins counting her seven clean days. If she saw only a single day, she must additionally keep a מוך דחוק (a tight cloth) present throughout בין השמשות. This is practical halacha one learns precisely, step by step, from a Rav and a moret kalla.
לכתחילה the woman performs two בדיקות per day during the שבעה נקיים — one in the morning (שחרית) and one close to בין השמשות. בדיעבד, a single בדיקה over the whole (the 1st day, the 7th, or a day in the middle) suffices, and one who increases the בדיקות is called praiseworthy (משובחת). The בדיקות (of the hefsek and of the seven days) are done with an old white linen cloth, cotton or white wool, clean and soft, inserted deep into the crevices, עד מקום שהשמש דש, then examined in daylight against any redness (אדמומית). When in doubt about a color, she never decides alone: she keeps the éd and shows it to a Rav (see siman 188).
The שבעה נקיים must be consecutive: if she sees blood during these days — even on the eve of the seventh day — it voids (סותר) the entire count, and she must redo a הפסק טהרה and recount seven days. For a mere כתם (stain), the Rama brings a יש אומרים that during the first three days one does not mitigate (תולין) as on the later days; but the nature and measure of a stain are judged case by case (see siman 190 on כתמים). In practice, faced with any stain or doubtful color during the seven days, the conduct is to put the question to a Rav, without delay, and never to decide alone.
The פולטת שכבת זרע — the emission of semen during the days of counting, within the six עונות following relations — voids that day of the count. Therefore, after relations, the woman begins counting her seven clean days only from the fifth day (יום ה' לשימושה), once these עונות have passed (see siman 193 for the בעילת מצוה). She is careful to clean well (with a מוך, a cloth or warm water) to expel the semen; but the Rama notes a יש אומרים that today we are no longer expert (אין אנו בקיאין) to rely on that cleaning. The exact count after relations, and the conduct to follow, are learned from the Rav and the moret kalla.