Yevamos, Chapter 4, Mishnah 10. This mishnah addresses how we are to treat a woman who has left her first marriage and is about to marry another man - whether she is a yevamah or any other woman - so that we will be able to clearly identify the father of any child she may bear.
The law of the yevamah:
"HaYevamah lo sachlotz v'lo sisyabem ad sheyesh lah sheloshah chodashim" - a yevamah does not undergo chalitzah and is not taken in yibum until three months have passed since her husband's death. There are two reasons for this:
Regarding yibum: we must verify that the yevamah is not pregnant from her deceased husband, for if she is pregnant she is forbidden to undergo yibum.
Regarding chalitzah: we do not want chalitzah performed either, since it is a rule that chalitzah is done only where yibum is a possibility. Furthermore, the chalitzah might ultimately turn out to be invalid, and people would mistakenly assume that a proper chalitzah had taken place. Therefore one must wait until three months have passed.
And likewise all other women:
"V'chein kol she'ar hanashim lo yisarsu v'lo yinasu ad sheyihyu lahen sheloshah chodashim" - any other woman as well, whose husband died or who was divorced from him, does not become betrothed and does not marry until three months have passed.
The prohibition against betrothal is merely an additional concern and a decree, for after betrothal the woman is still not secluded with her husband and there is no concern that she will become pregnant from him; but since marriage is forbidden, they decreed against betrothal as well.
The reason for all this: we want to know who the father is. The concern is not the child's legitimacy, since he is legitimate in any event, but rather knowing whether he is from the first husband or the second. The Gemara derives this principle from the verse teaching that the Divine Presence of the Holy One, blessed is He, rests only where it is known precisely who is related to whom.
"Echad besulos v'echad be'ulos, echad gerushos v'echad almanos, echad nesuos v'echad arusos":
"Echad besulos v'echad be'ulos" - whether she is a virgin from her first husband or whether he had already been intimate with her. We are strict even with a virgin, and she too may not marry immediately.
"Echad gerushos v'echad almanos" - whether the marriage was dissolved by a get or whether it was dissolved by the husband's death.
"Echad nesuos v'echad arusos" - whether she had full marriage with the first man or only betrothal with him.
It is worth noting that these are not six separate cases: the virgin is the one who had only betrothal, and the non-virgin is the one who had full marriage. It emerges that there are only four different cases here.
The view of Rabbi Yehudah:
Rabbi Yehudah is not as strict, and he distinguishes between betrothal and marriage:
Married women - may become betrothed: those who had full marriage with the first man, and who may be pregnant from him, are permitted to become betrothed to the second man even within three months, because he is not intimate with them during betrothal.
Betrothed women - may marry: those who had only betrothal with the first man, and who are certainly not pregnant from him, are permitted to enter the chuppah and marry the second man, since there is no concern here that the child is from the first man.
However, all this applies to betrothed women generally, but not to a betrothed woman in Yehudah. In Yehudah it was the custom to have the groom and bride secluded with each other even before the marriage, while she was still betrothed, so that they would become accustomed to one another. And although by law they were not supposed to be intimate with each other, the possibility exists that they in fact were, and if so she may be pregnant from the first man. It emerges that a betrothed woman in Yehudah does not have the status of a fully betrothed woman.
The view of Rabbi Yosei:
Rabbi Yosei says: "Kol hanashim yisarsu, chutz min ha'almanah, mipnei ha'evel". All women are permitted to become betrothed, since betrothal to the second husband does not lead to pregnancy, and as long as the marriage does not take place until after three months, betrothal is permitted. The exception is the widow, who may not become betrothed on account of mourning: during the first thirty days she is still in mourning for her husband, and she may not engage in the rejoicing that accompanies betrothal.
In summary: we learned in this mishnah that a yevamah does not undergo chalitzah and is not taken in yibum until three months have passed, and likewise all other women do not become betrothed and do not marry within three months, so that we can determine who the child's father is. This law applies to virgins and non-virgins, to divorcees and widows, to married women and betrothed women. Rabbi Yehudah is lenient and permits married women to become betrothed and betrothed women to marry - excluding a betrothed woman in Yehudah, and Rabbi Yosei permits all women to become betrothed, except for the widow on account of mourning.