The Mishnah presents four classifications of women in the context of marriage and levirate marriage (Yibum), outlining different scenarios where a woman may be permissible or forbidden to her husband (Baal) and/or her brother-in-law (Yavam).
The first classification details women who are permissible to their husbands but forbidden to their Yavam. For example, a Kohen Hedyot (a common priest) who marries a widow (Almana) acts permissibly, as a Kohen Hedyot is permitted to marry an Almana. However, if this Kohen Hedyot dies and has a brother who is a Kohen Gadol (High Priest), the Almana is forbidden to the Kohen Gadol for Yibum, as a Kohen Gadol may not marry an Almana. Another case involves a Kohen who has become a Halal (a disqualified priest) marrying a Kesherah (a woman fit for priesthood). This marriage is permissible. However, if the Halal has a brother who is a Kesher Kohen (a fit priest), the Kesherah, having married a Halal, becomes a Halalah (a disqualified priestess) and is therefore forbidden to the Kesher Kohen for Yibum. A third example is an Yisrael (an Israelite) marrying an Yisraelit (an Israelite woman), which is a regular, permissible marriage. If the Yisrael dies and has a brother who is a Mamzer (a bastard), the Yisraelit is forbidden to the Mamzer for Yibum. Finally, a Mamzer marrying a Mamzeret (a female bastard) is a legal marriage. If the Mamzer dies and has a brother who is an Yisrael, the Mamzeret is forbidden to the Yisrael for Yibum.
The second classification describes women who are forbidden to their husbands but permissible to their Yavam. For instance, a Kohen Gadol who performs Kiddushin (betrothal) with an Almana acts forbiddenly. However, if he dies and has a brother who is a Kohen Hedyot, the Almana would be permissible to the Kohen Hedyot for Yibum. Another case involves a Kesher Kohen marrying a Halalah. This marriage is forbidden for the Kesher Kohen. But if he dies and has a brother who is also a Halal, the Halalah is permissible to the Halal for Yibum. Similarly, an Yisrael who marries a Mamzeret acts forbiddenly. If he dies and has a brother who is a Mamzer, the Mamzeret is permissible to the Mamzer for Yibum. Lastly, a Mamzer who marries an Yisraelit acts forbiddenly. If he dies and has a brother who is an Yisrael, the Yisraelit is permissible to the Yisrael for Yibum.
The third classification covers women who are forbidden to both their husbands and their Yavam. An example is a Kohen Gadol who marries an Almana. This marriage is forbidden for the Kohen Gadol. Furthermore, because the marriage takes place, she becomes a Halalah, making her forbidden even to a Kohen Hedyot brother. Thus, she is forbidden to both the Kohen Gadol (due to being an Almana) and any Kohen brother (due to becoming a Halalah). Another instance is a Kesher Kohen who marries a Halalah, which is forbidden. If he dies and has a brother who is also a Kesher Kohen, the Halalah is also forbidden to this brother for Yibum. An Yisrael marrying a Mamzeret is forbidden. If he dies and has an Yisrael brother, the Mamzeret is forbidden to the Yisrael for Yibum. Finally, a Mamzer marrying an Yisraelit is forbidden. If he dies and has a Mamzer brother, the Yisraelit is forbidden to the Mamzer for Yibum.
The Mishnah then introduces the concept of Shiniyot, secondary forbidden relatives by rabbinic decree (Medivre Sofrim). It states that if a woman is a Shiniyah to the husband but not to the Yavam, she is forbidden to the husband but permissible to the Yavam. Conversely, if she is a Shiniyah to the Yavam but not to the husband, she is forbidden to the Yavam but permissible to the husband. If she is a Shiniyah to both, she is forbidden to both. Rashi illustrates a case where a woman is a Shiniyah to the husband but not to the Yavam. This can occur with paternal half-brothers who do not share the same maternal grandmother. If a man marries his maternal grandmother (Em Imo), this is a rabbinically forbidden marriage (a Shiniyah). If he then dies, and his paternal half-brother, who does not share this grandmother, is the Yavam, the grandmother is permissible to the Yavam. The reverse case, where she is a Shiniyah to the Yavam but not the husband, is also possible with paternal half-brothers. If the Yavam's maternal grandmother is married to the deceased brother, but she is not the Yavam's maternal grandmother, the marriage was permissible for the husband, but she is forbidden to the Yavam.
The Mishnah then outlines the consequences for marrying a Shiniyah: she loses her Ketubah (marriage contract payment), the husband does not have to return the profits (Perot) from her assets (Nichesei Milog), and she is not entitled to support (Mezonot) from him. The reason for the forfeiture of Ketubah and Perot is that the Sages imposed a penalty (Kenas) because the marriage was forbidden. The husband is not obligated to provide support because he is required to divorce her. Even if she borrowed money for support while he was away, he is not liable to repay the loan. However, the children born from such a union are considered legitimate (Kasher). Finally, the husband is compelled to issue a divorce (Get).
The Gemara questions the Mishnah's choice of examples. Regarding the first case of a Kohen Hedyot marrying an Almana, which is permissible to the husband but forbidden to the Yavam (Kohen Gadol), the Gemara asks why the Mishnah specified Nisuin (full marriage) rather than Kiddushin (betrothal). The halachic status of the Almana being forbidden to the Kohen Gadol would be the same even with just Kiddushin. The Gemara answers that the Mishnah chose Nisuin to align with a later case in the Mishnah (the Sefah), where a Kohen Gadol marries an Almana, making her a Halalah and thus forbidden to all Kohanim. This transformation into a Halalah only occurs through Nisuin, not Kiddushin. Therefore, to ensure the Sefah's ruling holds, Nisuin is necessary. However, the Gemara retracts this, noting that this specific Sefah case is about a woman forbidden to both the husband and Yavam, not the second classification (forbidden to husband, permissible to Yavam) where the Kohen Gadol marries an Almana and has a Kohen Hedyot brother. Instead, the Gemara explains that the Mishnah chose Nisuin for the first case because of the next case (Bat Bukta) in the same classification: a Halal marrying a Kesherah. In this instance, the Kesherah becomes a Halalah only through Nisuin, making her forbidden to a Kesher Kohen brother. Since this subsequent case requires Nisuin, the preceding case also uses Nisuin for consistency within the grouping.
The Gemara further asks why the Mishnah chose an Almana in the first case. Why not a Betulah (virgin) who marries a Kohen Hedyot, dies, and then becomes an Almana, forbidden to the Kohen Gadol brother? The Gemara initially suggests this is because of the principle of Nisuin Rishonim Mapilin, which posits that the status for Yibum is determined at the time of the first marriage. If so, a Betulah would have been permissible to the Kohen Gadol at the time of her marriage to the Kohen Hedyot. However, the Gemara refutes Nisuin Rishonim Mapilin based on the case of a Halal marrying a Kesherah, where she becomes a Halalah only after the marriage, yet is forbidden to the Yavam. Therefore, the status is determined at the time of Yibum, not the initial marriage. The Gemara concludes that the Mishnah chose Almana for consistency with the Sefah, where a Kohen Gadol marrying an Almana is forbidden to both the Kohen Gadol and any Kohen Hedyot brother (due to her becoming a Halalah). A Betulah would not create this dual prohibition in the Sefah, so Almana is used throughout.
Rav Papa then challenges the Mishnah, arguing that it could have presented scenarios involving Mitzrim (Egyptians) that would fit all four classifications. For example, a Mitzri Sheni (second-generation Egyptian convert) marrying a Mitzrit Rishonah (first-generation Egyptian convert) is a legal marriage, and their child is a Mitzri Sheni. If a Mitzri Sheni marries a Mitzrit Rishonah and a Mitzrit Sheniyah (second-generation Egyptian convert), and has brothers who are a Mitzri Sheni and a Mitzri Shlishi (third-generation Egyptian convert), various combinations would yield all four types of permissibility/prohibition for husband and Yavam. The Gemara initially responds that the Mishnah does not need to list every single case. However, if the Mishnah omits one case, it should omit others to demonstrate that its examples are not exhaustive. The Gemara then suggests that the Mishnah could have omitted cases involving a Pesua Daka (a man whose testicles are crushed), which also fits all four categories. The Gemara rejects this, stating that Pesua Daka is a type of Yisulav (one disqualified from marrying into the congregation), and the Mishnah already provided models for Yisulavim (e.g., Mamzerim, Halalim). Therefore, Pesua Daka is implicitly included and not considered an omitted case.
The Gemara then asserts that the cases listed in the Mishnah are all necessary, not redundant. The case of a Kohen Hedyot marrying an Almana is needed to teach about a prohibition based on a negative commandment (Lo Ta'aseh). The case of a Halal marrying a Kesherah is needed to teach that a Kohenet (priestess) is permitted to marry a Halal. The Gemara clarifies that the term Kesherah in this context refers to a Kohenet, demonstrating that such a marriage is permissible for the husband. If Kesherah referred to an Yisraelit, there would be no novelty. The Gemara then addresses a third case, an Yisrael marrying an Yisraelit whose brother is a Mamzer. This case is needed to teach about a prohibition that applies to everyone (Lav Shehaveh Lakol), as opposed to a prohibition that only applies to Kohanim (Lav She'eino Shehaveh Lakol), like the Halalah. Both types of prohibitions lead to the woman being forbidden for Yibum. Since all the Mishnah's cases are necessary, the Gemara concludes that the Mishnah did not omit Pesua Daka as it is covered by the existing models. However, it still maintains that the Mishnah did omit the Mitzri cases.
The Gemara then revisits the ruling that a Kohenet is permitted to marry a Halal. Rav Nachman challenges this from the double "Lo Yikahu" (he shall not take) in the verse "Isha Zona VeHalala Lo Yikahu." This double negative is interpreted to teach that just as a Kohen is forbidden to marry a Halalah, so too a Kohenet is forbidden to marry a Halal. Rava responds that the double "Lo Yikahu" teaches that both the man and the woman incur a prohibition in a forbidden marriage. It does not teach about Kesherot (fit women) marrying Halalim, but rather that in the case of a Kohen marrying a Halalah, both parties are culpable. The Gemara then questions Rava's explanation, arguing that the principle of equal culpability for men and women in negative commandments (Lo Ta'aseh) is already derived from the verse "Ish O Isha Ki Ya'asu Mikol Hatot Ha'adam." Rava clarifies that "Ish O Isha" only teaches equality for prohibitions that apply to everyone (Shaveh Lakol). For prohibitions that are not universal, like a Kohen marrying a Halalah, the double "Lo Yikahu" is necessary to teach that both parties are culpable. The Gemara then considers an alternative interpretation: "Ish O Isha" could apply to all prohibitions, even those not universal. If so, a special derivation would be needed to exempt Kohanim's daughters (Benot Aharon) from the laws of ritual impurity (Tumah), as the verse "Bnei Aharon" (sons of Aharon) implies, and without it, they would be included by "Ish O Isha." The Gemara concludes that "Ish O Isha" indeed applies only to universal prohibitions, and the double "Lo Yikahu" is specifically for non-universal prohibitions where both parties are culpable. Thus, the original premise that a Kohenet can marry a Halal remains unchallenged by this verse.