Massekhet Yevamot is the first tractate in Seder Nashim. While Massekhet Yevamot focuses on the rules and regulations concerning yibum it nevertheless involves an examination of forbidden and disqualified sexual and marital relationships, as well as the definition and severity of those prohibitions. Thus, the comprehensive discussions of Massekhet Yevamot form the basis of the entire Seder Nashim.
The mitzva of yibum appears only once in the Torah ( 25:5-10); however, its application – which is not an uncommon occurrence – involves so many general concepts of Jewish marriage and relationships that an entire tractate is devoted to its study.
Conceptually, the mitzva of yibum can be understood as follows:
When a man passes away having brought no offspring into the world, it leaves a vacuum in the soul of the individual who has not succeeded in fulfilling his life’s goals. The surviving brother – the yavam – is called upon to replace his brother who has passed on and, by marrying his wife, effectively takes that brother’s place. For this reason, the Torah does not require any formal ceremony for yibum, as this marriage is not seen as a new relationship, but rather as a continuation of an already existing one. Ideally, this relationship will bring children into the world, thus fulfilling the life goals of the dead brother.
In the event that the yavam and yevama choose not to carry on the relationship, the Torah calls for a ceremony of atonement and purification – known as halitza – to take place. Not only are the yavam and yevama freed of their mutual obligations by means of this ceremony, but the circle of life of the deceased brother is marked as coming to a close, as well.
The basic foundation of yibum is unusual, in that a woman who was married to a man who dies with no offspring is permitted to marry one of his brothers, and, in fact, is performing a mitzva in doing so. This rule stands in opposition to the law forbidding someone from marrying a woman who is married to his brother (see Vayikra 18:16 and 20:21), one of the issurei erva – forbidden sexual relations – for which the punishment is karet, one of the most severe penalties in the Torah.
The mitzva of yibum is unusual not only because of its being an apparent exception to the rules of issurei erva, but also because the Torah offers an option that allows an alternative to fulfilling this mitzva. Should the surviving brother decide that he does not want to marry his sister-in-law, he can choose to perform halitza, and she will then be free to marry outside of the family. Generally speaking, we find mitzvot in the Torah that are either obligatory under all circumstances (like the commandment to love God or to lay tefillin), or else under specific circumstances (like rules of business or slaughtering animals for food). Once a person is obligated, however, he must carry out the mitzva as commanded. Thus, the Sages do not understand the option of halitza as negating the mitzva of yibum; rather, it is presented as an alternative mitzva, and according to one opinion (Yevamot 39b) the preferred mitzva.
Another out-of-the-ordinary aspect of yibum is the unusual relationship that is created. Under normal circumstances a man and woman choose to marry one another by their own free will. In the case of yibum, the man and woman involved find themselves in a relationship imposed upon them by the Torah. An analysis of that relationship – referred to by the Sages as zikah – is one of the fundamental questions taken up in Massekhet Yevamot. While some of the Sages view zikah as simply a preparatory state that exists until the yavam chooses to perform either yibum or halitza, the conclusion of the Gemara is that it is almost a form of marriage, with the woman seen as engaged to her late husband’s brother.
To clarify intentions, the Sages required the yavam who plans to fulfill the mitzva of yibum to perform an act of kiddushin – referred to as ma’amar – prior to performing the mitzva. Similarly, they give a certain amount of credence to a geṭ given by the yavam to his sister-in-law, even though it is only through halitza that she will become free to marry outside the family.