Yevamot יבמות

If a man died childless, his brother must either marry the widow or release himself by the rite of chalitzah. From this comes the web of kinship the tractate takes up in its very first mishnah, which lists the women who exempt both themselves and their rivals from levirate marriage.

The last chapters are about the agunah, the woman whose husband has not returned and of whose death there is no reliable testimony. For her sake the sages made a rare relaxation in the rules of evidence: a single witness is accepted, and a woman, and hearsay. Strictness here turns into cruelty, and the law admitted it.

Order: Nashim. There is Gemara in both the Babylonian and the Jerusalem Talmud.