Kiddushin, Chapter One, Mishnah One
The first mishnah of Kiddushin teaches how a woman is acquired in marriage and how she acquires herself, that is to say, how she becomes free to marry another man. The mishnah also teaches how a yevamah is acquired. A yevamah is a woman whose husband has died without any children (see the intro to tractate Yevamot). According to the Torah she must either marry her husbands brother or perform halitzah, the release from the obligation to her brother-in-law.
1) A woman is acquired in three ways and acquires herself in two:
a) She is acquired by money, by document, or by intercourse.
b) By money:
(1) Bet Shammai says: a denar or the equivalent of a denar;
(2) Bet Hillel says: a perutah or the equivalent of a perutah.
(a) And how much is a perutah?
(b) An eighth of an Italian issar.
2) And she acquires herself by divorce or by her husband’s death.
3) A yevamah is acquired by intercourse.
4) And she acquires herself by halitzah or by the yavams death.
Section one: A man can betroth his wife in any one of three ways. The first is by giving her a small amount of money and saying to her Behold you are betrothed to me with this money. Bet Shammai and Bet Hillel debate how much money is needed to effect betrothal. What is crucial is that both a denar and a perutah are small amounts of money; a perutah is almost valueless. These are not representative of a womans true value, which is clearly much greater. Rather they are symbolic, especially in Bet Hillels opinion. To this day, nearly all betrothals are effected through money. Since the Middle Ages and perhaps earlier, Jews have used rings to effect betrothal. This custom was originally a Christian custom. In the Talmud rings are never used.
The second way is for the husband to write her a document in which it is stated, Behold you are betrothed to me. This document is not to be confused with a ketubah, although some scholars posit that they are both derived from common origins and that originally they were written together.
The thirds means of betrothal is sexual relations. This act must be done with the intent of betrothal. No one holds that casual intercourse can effect betrothal. The thornier problem is whether or not sexual relations between a couple living together can effect betrothal. Most modern halakhists rule that it does not, although there are some who hold that couples who live together with the intent to form a familial type of unit do require a get in order to separate.
Section two: A woman becomes halakhically separated from her husband either by divorce or by death. Without one of the two, any relations that she has with another man will be considered adultery.
Section three: The dead husbands brother-in-law acquires his brothers widow through sexual intercourse. As we learned in Yevamot, the yevamah is not acquired by money, as a woman would be in cases of normal betrothal. However, the rabbis added on that before the couple has sexual relations, the yavam should perform an act of betrothal through money, as is done in normal cases. This act of betrothal does not have toraitic (deoraita) legal consequences.
Section four: The yevamah is free to marry another man if she performs halitzah with the yavam. Alternatively, if the yavam dies (in a situation where there is only one yavam) she also may marry anyone she so chooses. Note that once she is married she is considered a normal wife, and she acquires herself through the death of her husband or through divorce.