The ninth perek of Massekhet Yevamot begins on today’s daf, and offers a list of relationships, including women who are:
permitted to their husbands, but not to their yavam (e.g. a widow who is married to a regular kohen, whose brother is the kohen gadol),
permitted to their yavam, even though they were forbidden to their husbands (e.g. a widow married to the kohen gadol whose brother is a regular kohen),
forbidden to both (e.g. a regular woman who is married to a mamzer, whose brother is a mamzer, as well),
permitted to both (e.g. most normal cases of marriage).
The Ritva points out that the idea that even people whose initial marriage was problematic – but valid – would still be obligated in the mitzva of yibum (levirate marriage) comes as something of a surprise, since we could have argued that it was not a marriage that was meant to produce children. Still, the Mishna teaches that even though the first husband was obligated to divorce his wife, nevertheless, since the marriage does have halakhic validity, yibum (or halitza – a ceremony releasing one from the obligation of yibum) will be necessary should the first husband pass away.
Although the last clause of the Mishna states that in “all other cases” women are permitted to both their husbands and to their yavam, the Talmud Yerushalmi points out that there are exceptions to this rule that are not enumerated in the Mishna. Based on this observation, the Yerushalmi concludes that kelalav shel Rebbi einam kelalim – general principles presented by Rabbi Yehuda HaNasi, who authored the Mishna, cannot be relied upon as definitive – since there are invariably exceptions to the rule.
Tosafot explain that this perek synopsizes the relationships – both permitted and forbidden – that yevamot may find themselves involved with, since the continuation of Massekhet Yevamot focuses on other issues that are connected with the mitzva of yibum, but no longer with relationships.