Siman 195

The sages taught in the Mishnah (50a): Two groups that were eating in one house, when some of them see one another—they join together for zimun. And if not—these perform zimun for themselves, and those perform zimun for themselves. And we say in the Yerushalmi: It was necessary for two houses. That is to say: for regarding one house it is not relevant to say "when some of them see one another," for in one house, do they not all see each other? Rather, certainly there is no difference between one house and two houses. And the Mishnah, which used the term "one house," informs us that even in one house they may perform zimun these for themselves and those for themselves when there are ten in each group, or when even together there would not be ten. For that which was taught "they join together for zimun"—certainly it is not a necessity that they join, and if they wish, they may perform zimun each and every group by itself (and so wrote the Bach). And for this it informs us that even in one house they may divide, as I have written in siman 193. And it is simple that they all must be able to hear from the mouth of the one reciting the berakhah (ibid.).

And it was taught there that the attendant also joins them, meaning: even if these do not see those, the attendant joins them if he serves both groups, and even if they are in two houses. And if a public domain separates the two houses – they do not join in any manner, even if these see those, and even if one attendant serves both of them. And it is not specifically a public domain, but even if a private path separates them – they do not join (Taz, subsection 2). But at a wedding feast, a public domain does not separate, since the whole is a wedding feast (ibid. subsection 3). And more than this was explained in Even HaEzer siman 62, that even if these do not see those, and one attendant does not serve them, nevertheless they join, see there. And so wrote the Tur here in the name of the Rosh, see there.

In siman 193, section 12, we explained that those who eat at two tables do not join together. And that which two groups join together, this is when there is a zimun in each and every group; see there. However, even for this we do not need, for the Tur and the Shulchan Aruch wrote here, for example, that they entered from the beginning with the intent to join together. And this is according to the Yerushalmi. Therefore, we properly join one to another even when they sit at two tables. And it is possible that in such a case they must necessarily join, unlike what I wrote in section 1. However, it is impossible to say so, for even in one group they may divide if they wish, when there is a zimun for each one or ten, as I wrote there.

And according to this, a new law emerges for us: that which was explained there, that on two tables they do not combine for zimun, if they entered from the beginning for that purpose—they combine. And even if there are not three at each table—they are obligated in zimun. And that which we wrote there, that when there is a zimun in each group they combine—it is not certain, and the custom is to combine. And one must deliberate on this regarding the law. (See the Beit Yosef who raised a difficulty: for the Rosh holds that even joining at the end is effective, as I wrote in siman 193. And it is not a difficulty at all: for here even at the end there is no joining. And so wrote the Bach and the Magen Avraham, subsection 2. And see the Taz, subsection 1, who elaborated, and there is no need for this at all. And the Beit Yosef in his great book wrote because of this difficulty: that we do not require that they entered with the intent to combine together except in two houses and not in one house, see there. And according to this, we will need to say as I wrote there, that when there is a zimun at each table—it combines. And this is the intent of the Magen Avraham at the end of subsection 2, see there. But from the language of the Tur and Shulchan Aruch it implies explicitly that it also refers to one house, as the commentators wrote. And according to this, there is no need for what I wrote there. Examine it and you will find it simple.)

If some of them ate inside the house and some of them outside the house, if the one reciting the blessing sits on the threshold of the house and he sees these and those—he joins them, for it is as if some of them see each other. Our teachers, the authors of the Shulchan Aruch, wrote in section 3: Wherever two groups join together—both must hear the words of the one reciting the blessing of zimun clearly. And the rest of Birkat HaMazon each one shall recite for himself. But if they desire that the one reciting the zimun fulfill the obligation for everyone—they must hear the entire Birkat HaMazon, for without this they have not fulfilled their obligation at all. Thus far his words. And if one does not hear, he answers "Amen" (Magen Avraham, subsection 5). And several poskim wrote that in any case they must hear the entire Birkat HaMazon from the one reciting the blessing (ibid., subsection 4). And in siman 193, section 8, we explained this, see there.