The Obligation of Zimmun For Those Who Eat While Standing
Does the obligation of zimmun apply to people who eat while standing?
The Mishna in Berakhot states (42a):
If [those at the table] are sitting upright, each one recites the blessing for himself; if they have reclined, one recites the blessing for all.
Rashi (ad loc.) comments:
If they were sitting upright without reclining on couches – i.e., reclining on their left side on a couch and eating and drinking in a reclined position – each one recites the blessing for himself, for there is no fixed meal without reclining.
The Gemara (ibid. 42b) explains that if they prepared for themselves a place to eat, e.g., if they said: "Let us go and eat bread in such and such a place," even if they have not reclined, one recites the blessing for all. The Rif (31a) cites this ruling of the Mishna and the Gemara with respect to one person reciting the ha-motzi blessing on behalf of others. The Rambam (Hilkhot Berakhot 1:12) also cites this ruling only in that context. The Tosafot (s.v. hesebu), on the other hand, prove that the Mishna refers to both the ha-motzi blessing and birkat ha-mazon. This approach is also adopted by the Rosh (7:1), Talmidei Rabbeinu Yona and the Rashba. It is clear according to them that in a case where each person recites ha-motzi for himself, and one cannot discharge the obligation of another – there is no requirement of zimmun. What is more, the Magen Avraham (193:8) goes further and says that in a case where one person cannot discharge the obligation of another, not only is there no obligation of zimmun, but zimmun is not permitted. The Mishna Berura (no. 24) writes regarding this:
Even though there are certain Acharonim who have rejected his proof regarding zimmun, nevertheless his position is the most reasonable.
And as he explains there in Sha'ar ha-Tziyun:
For how can they say, "let us bless," the intention being that we should recite the blessing together, when in the end they must divide up, since one person cannot discharge the obligation of another.
As for the Halakha, the Shulchan Arukh (193:3) rules in accordance with Tosafot; see the Mishna Berura there (no. 25) who explains that the parameters of fixity (kevi'ut) stated with respect to the ha-motzi blessing (167:12) apply equally with respect to joining for zimmun.
As for the definition of fixity, Tosafot say:
Our sitting establishes fixity like their reclining, for they were accustomed for each person to recline on his couch and at his table. But today we all eat at one table, and when we eat together, that is our fixity… Nevertheless, people who eat on the road, while walking rather than seated - each one recites the blessing for himself.
We see then that Tosafot distinguish between walking and sitting. Regarding how this would apply to standing, one could draw inferences from this statement in either direction (standing being less fixed than sitting but more than walking). Nevertheless, it would seem that Tosafot maintain that standing is like sitting. For if their position is that there is no fixity without sitting, why do they go into details about people who are on the road? Let them simply say that those who eat in a standing position, which is a common everyday situation – each one recites the blessing for himself. Yet, the wording of Tosafot, which speaks of sitting around a joint table, requires further clarification on this approach. It might be possible to explain the wording of Tosafot in light of the Shulchan Arukh's ruling (167:11-12):
Today when we are not accustomed to recline, our sitting at one table or without a table at one cloth constitutes fixity, and it is like their reclining as members of a group. And for us, even if a group of people fixed a place for their meal, or the head of a house arranged to sit with the members of his household, it does not help unless they sat down together at one table or at one cloth.
If they were riding, and they said: "Let us eat," even if each person ate from his own loaf, because they did not dismount from their animals, they join together, since they were standing in one place.
The Magen Avraham (no. 26) asks: In the first ruling cited, the Shulchan Arukh said that a verbal declaration does not establish fixity, unless they actually sat at one table, whereas in the latter ruling he states that the statement “Let us eat” is sufficient. He answers that it is only in the house that we require one table, but out in the field, where there are no tables, fixity can be established through speech, even without a table.
According to this, it might be argued that this is the reason that Tosafot emphasize: "Nevertheless, people who eat on the road, while walking rather than seated." They mean to say that despite the fact that on the road there is no need for the group to sit around one table, for there is no table there, and one might have that even sitting is not needed – nonetheless, since they did not sit, each person recites a blessing for himself. But in a house, if people were standing and eating at one table, there is fixity even without sitting. And it should not be objected that Tosafot had earlier emphasized that in our day when there is no reclining, our sitting constitutes fixity, implying that sitting is a necessary condition for fixity – for their primary emphasis is on eating at one table, and generally we eat while sitting down. But it is still possible that if people are eating at one table, then there is fixity even if they are standing. Support for this approach may be brought from the wording of the Rabbeinu Yitzchak cited by Talmidei Rabbeinu Yona (30b in the Rif’s pagination):
Rabbeinu Yitzchak further said that in their days when there were [separate] tables and cloths in front of each person, they had to recline in order for all to join together. But today when it is our custom all to eat at one table and with one cloth, there is no need for reclining, but rather through this alone they join together. Even though each person eats from his own loaf, one recites a blessing for all, both with respect to the ha-motzi blessing and with respect to birkat ha-mazon.
There is no mention here of a requirement of sitting. The Bi'ur Halakha, however, writes (167, s.v. yeshiva didan):
But standing does not help (Tosafot, Berakhot 43a, s.v. ho'il). And the ruling cited in par. 12 where standing suffices is a special case, for there they are sitting on donkeys.
However, it appears that the proof is cites from Tosafot is questionable. The Tosafot state as follows:
But we must further examine the case of havdala, for we recite havdala while standing. How then does one person discharge the obligation of another with respect to the blessing over the wine and the havdala, as we are not seated or in a reclined position? It might perhaps be suggested that since we fix ourselves in order to fulfill the obligation of havdala, this is effective with regard to all of its aspects. Therefore it is good and proper that the person reciting havdala and also those who hear it should sit for havdala, so that it should look like a fixed situation, and he can exempt them.
It seems to me, however, that there is no proof from here that sitting is required even when people are all eating from the same table, for the reason that a table can join people together stems from the fact that they all eat from it. But in the case of havdala, where there is no eating, there is nothing to join them, and therefore we require sitting. But where people actually eat jointly from one table, only that they eat while standing, it is possible that the fact that they are standing does not interfere with their fixity. So too, it cannot be proven from here that even when they designate a place by saying, "Let us eat here," sitting is still required, for with respect to havdala, we don't ordinarily declare, "Let us make havdala here," and therefore Tosafot require reclining or sitting. But in a place where fixity can be established through speech, it is possible that we do not require sitting.
With regard to eating jointly, the Ma'amar Mordekhai (167:12) proves that there is no need to say, "Let us eat here," and that "Let us eat" suffices, as opposed to what may be implied by the wording of the Posekim, that it is necessary to spell out the place. According to this, it is possible that even in the case of havdala we are dealing with a case where the fixity is established through the words of the person reciting havdala, or through the verses of blessing recited prior to havdala, as is our custom (and this is a longstanding custom, cited already in the Machzor Vitri). It is possible that the recitation of these verses is treated like a call to join the person reciting havdala. In any event, it seems that Tosafot are dealing with a case where the fixity is not established through words, for they ask about our custom not to sit or recline, and if we are dealing with a case where the fixity is established through words, there would be no need for reclining.
In any event, as for the Halakha, one must not veer from the Mishna Berura's ruling that there is no fixity without sitting. This also seems to be the view of the Vilna Gaon in his commentary, for he disagrees with the aforementioned Magen Avraham who distinguishes between a house and a field with respect to one table, and it is his position that there is no difference, only that if they designated a place with words, there is no need to be seated at one table. As for what the Shulchan Arukh writes (167:11) –
And for us, even if they fixed a place for their meal, or the head of a house with the members of his household, it does not help unless they sat down together at one table or at one cloth -
the Vilna Gaon explains that the words "at one table or at one cloth" are simply a manner of speaking, and there is no need to sit at one table, only to sit. This is also the implication of the Beit Yosef who doesn't mention the requirement to sit at one table in this context. According to this, even in our day, though the law of fixity through words has not been canceled, the Shulchan Arukh nevertheless requires sitting, and this must be because he maintains that there is no fixity without sitting. According to this, even if people are eating around one table, there is no fixity without sitting.
What follows from the above discussion (and additional laws regarding zimmun) is as follows:
1) When people eat standing up, even from one table, there is no zimmun, even without mentioning God's name.
2) When people eat in a moving vehicle, there is zimmun, but no mention of God's name, based on the uncertainty, even if ten men are present.
3) When people eat in a stationary vehicle, and not out of a shared carton which could be treated like a table or a cloth, care should be taken that it be stated at the outset, "Let us eat here," and then zimmun can be recited, even with God's name.
4) When people eat in a car, with each person faced in a different direction, there is no zimmun at all.
5) When they eat dispersed in a field, or even close together, only that each person faces a different direction, there is no zimmun even if they designated the place from the outset, saying, "Let us eat here," and even if they ate from the same loaf.
(Translated by David Strauss)
 Unlike the view of the Ramban that there is no law of fixity with respect to zimmun. In my opinion, this is also the view of the Rif, see there, and in the Ra’avya (I, 119).