Yevamos, Chapter 9, Mishnah 2. In the previous Mishnah we listed a series of women who are permitted to their husbands and forbidden to their yevamim. Our Mishnah deals with the reverse case: women who are permitted to their yevamim and forbidden to their husbands.
Women who are permitted to the yavam and forbidden to the husband:
A Kohen Gadol who betrothed a widow: A widow is forbidden to a Kohen Gadol, and therefore she is forbidden to her husband; but if he has a brother who is an ordinary kohen, who is permitted to marry a widow, she is permitted to the yavam. The Mishnah deliberately used the language of betrothal, because had the Kohen Gadol actually married her and lived with her as his wife, she would have become a chalalah through the kohen's forbidden relations, and then she would have been forbidden not only to a Kohen Gadol but even to an ordinary kohen. Only as long as she is merely a widow, and not a chalalah, does she remain permitted to the ordinary kohen.
A kohen of valid lineage who married a chalalah: She is forbidden to him, but if he has a brother who is a chalal, the chalalah is permitted to him.
A Yisrael who married a mamzeres: The mamzeres is forbidden to her husband, but to the yavam, who is a mamzer, she is permitted.
A mamzer who married a bas Yisrael: She is forbidden to her husband, who is a mamzer, and permitted to the yavam, who is a Yisrael.
In all of these cases the woman is permitted to her yavam and forbidden to her husband.
Women who are forbidden to the husband and to the yavam alike:
A Kohen Gadol who married a widow: Through the consummation of the marriage she becomes a chalalah. If he has a brother who is a Kohen Gadol, she is forbidden to him both as a widow and as a chalalah; and even if the brother is only an ordinary kohen, she is forbidden to him, since she became a chalalah through her relations with a Kohen Gadol while she was a widow.
A kohen of valid lineage who married a chalalah and has a brother who is also a kohen of valid lineage: She is forbidden to both of them.
A Yisrael who married a mamzeres and has a brother who is a Yisrael: The husband and his brother are both of valid lineage, and the mamzeres is forbidden to both of them.
A mamzer who married a bas Yisrael and has a brother who is a mamzer: She is forbidden to both of them.
The conclusion of the Mishnah:
The Mishnah concludes: "Ushe'ar kol hanashim mutaros le'ba'aleihen veliyvameihen" - and all other women are permitted to their husbands and to their yevamim. The Gemara clarifies that this statement is not precise and is not meant literally, for there are additional women who are permitted to their husbands and forbidden to their yevamim, such as where the yavam is a petzua daka, or where the woman is one of the arayos listed at the beginning of the tractate, and the like.