Pesachim, Chapter 8, Mishnah 1. At the outset we need to establish the basic law: a person may slaughter a Pesach offering and include others with him, and the rules of including them vary according to their status:
His minor children, and his Canaanite slave and maidservant - they are not full Jews, and therefore he may include them whether they are aware of it or not. Their consent is not needed, nor is their knowledge.
His adult children, and his Hebrew slave and Hebrew maidservant - they are full Jews, and therefore their consent is required: they must agree to be included in his Pesach offering.
Our Mishnah opens with a discussion of someone who is attached to two different people - which of them is he counted with?
A wife - her husband's offering or her father's:
"Ha'ishah bizman shehi b'veis ba'alah, shachat alehah ba'alah v'shachat alehah avihah - tochal mishel ba'alah" - a woman living in her husband's home, and both her husband and her father slaughtered a Pesach offering on her behalf, eats from her husband's, since from the moment she married she is attached to her husband.
"Halchah regel rishon la'asos b'veis avihah, shachat alehah avihah v'shachat alehah ba'alah - tochal bamakom shehi rotzah" - if the first Yom Tov after her marriage is Pesach, and she goes to spend it in her father's home as is the accepted custom, and both of them slaughtered on her behalf, she may eat in whichever of the two places she wishes.
The Gemara clarifies two points:
The decision at the time of the slaughtering: although she may eat wherever she wishes, she must decide already at the time of the slaughtering which of them she is being counted with. Otherwise the matter would depend on the principle of 'bereirah' - a retroactive decision - and we do not apply bereirah to a Torah law, and the Pesach offering is a Torah law.
A woman accustomed to going to her father's home: the case is one of a woman who was not accustomed to going to her father's home before that first Yom Tov. But if she used to go there regularly, and she went to her father's home for the first festival, she eats from her father's. On the second festival, however, when both of them slaughtered on her behalf, whether or not she is accustomed to going to her father's home, the law reverts to that of our Mishnah: she eats in whichever place she wishes.
An orphan for whom guardians slaughtered:
"Yasom sheshachtu alav apotropsin - yochal bamakom sheyirtzeh" - a minor orphan for whom two guardians were appointed, and both of them slaughtered on his behalf, eats wherever he decides; and since he is a minor, he need not decide at the time of the slaughtering, as was required of the woman in the previous case. But an orphan who is an adult, past the age of bar mitzvah, must decide at the time of the slaughtering where he will be. And if he did not decide, and his intention was toward both of them, he is counted with whichever one slaughtered first.
A slave belonging to two partners:
"Eved shel shnei shutafin - lo yochal mishel shneihem" - a Canaanite slave who belongs jointly to two owners. Although a Canaanite slave does not require consent, here he faces two different intentions - those of the two owners - and therefore he cannot eat from either of them.
The Gemara explains that the case is one of partners who are particular with one another, who do not get along and do not wish to benefit from each other. But if they are not particular, the slave may decide which of them to join. In our case, where they are particular, the partners should have agreed between themselves exactly where the slave would eat.
One who is half slave and half free:
"Mi shechetzyo eved v'chetzyo ben chorin - lo yochal mishel rabo" - for example, a slave who had two owners and one of them freed him, so that he is half free. He does not eat from the master who still holds his other half, because we assume that the master agreed to include him in the Pesach offering only for the portion that he owns, and not for the portion that is free. And since that portion is not included in the offering, he cannot eat from it.
The Gemara adds: although he does not eat from the master who owns half of him, he may eat from a Pesach offering of his own, since the law is that the master is compelled to free him completely. Therefore, even though he has not yet actually been freed, he is regarded as a free man with regard to eating from a Pesach offering of his own.
In summary: in this Mishnah we learned the laws of one who is attached to two people: the married woman, who eats from her husband's offering, whereas on the first festival, which she went to spend in her father's home, she eats wherever she wishes, provided that she decides at the time of the slaughtering because of the issue of bereirah; the orphan for whom his guardians slaughtered, and the distinction between a minor and an adult; a slave of two partners who are particular with one another, who eats from neither of them; and one who is half slave and half free, who does not eat from his master's offering but does eat from a Pesach offering of his own.