Gemara

Question: How far does Birkas ha'Zimun extend?

Answer #1 (Rav Nachman): It is until "Nevarech."

Answer #2 (Rav Sheshes): It is until (and including) Birkas ha'Zan (Es ha'Kol).

Suggestion: They argue like the following Tana'im:

(Beraisa #1): Birkas ha'Mazon is two or three (Berachos; alternatively, up to three people may say it);

(Beraisa #2): It is three or (alternatively - up to) four.

We are thinking that all agree that ha'Tov veha'Metiv is mid'Rabanan (so it is not counted). Tana #1 holds that Birkas ha'Zimun is until Birkas ha'Zan. Tana #2 holds that Birkas ha'Zimun is until Nevarech.

Rejection: No. Each Amora explains that both Tana'im agree with him;

Rav Nachman says that all agree that Birkas ha'Zimun is until Nevarech;

We understand Beraisa #2 (as explained above);

Beraisa #1 discusses [a shortened Birkas ha'Mazon for] workers;

A worker says Birkas ha'Zan, and combines Birkas ha'Aretz and Boneh Yerushalayim into one Berachah.

Rav Sheshes says that all agree that Birkas ha'Zimun is until Birkas ha'Zan;

We understand Beraisa #1 (as explained above);

Tana #2 holds that ha'Tov veha'Metiv is mid'Oraisa, therefore he counts one more.

48b (Beraisa) Question: What is the source for Birkas ha'Mazon mid'Oraisa?

Answer: "V'Achalta v'Savata u'Verachta" refers to Birkas ha'Zan; "Es Hash-m Elokecha" refers to Birkas ha'Zimun...

Rishonim

Rif (33b): If three ate together and one needed to leave, Rav Nachman and Rav Sheshes argue about how much he must hear before leaving. Rabanan had difficulty with the Sugya, for they understood that two, three and four refer to Berachos. If so, why does it say "Shnayim, Sheloshah, Arba'ah" (masculine forms of the numbers? "Berachah" is feminine!) I resolved it based on the Yerushalmi. Bei Rav said that one who must leave says the first Berachah before leaving, i.e. Birkas ha'Zimun, i.e. ha'Zan. Rav Sheshes asked from a Beraisa that says two or three. I.e. if two or three ate and none knows how to bless all of Birkas ha'Mazon, but for each Berachah, there is someone who knows it, each says the Berachah he knows aloud, and through this they are Yotzei through the two or three of them. The Beraisa taught that it can be split among at most three people. We infer that with Birkas ha'Zimun, Birkas ha'Mazon is only three Berachos. Another Beraisa says four. Rav holds like that Tana. Do not say that it says four due to ha'Tov veha'Metiv, for it is not mid'Oraisa, and the Tana would not count it with (the number needed for the) Berachos mid'Oraisa. Also the Bavli is like this. The Halachah follows Rav Nachman, who says until Nevarech. This is the custom, that an individual begins from Birkas ha'Zan. This shows that Birkas ha'Zimun is only until Nevarech.

Rambam (Hilchos Berachos 5:2): If three ate bread together, they must bless Birkas ha'Zimun. I.e. one of them says "Nevarech she'Achalnu mi'Shelo", all answer "Baruch she'Achalnu mi'Shelo uv'Tuvo Chayinu", and he repeats this.

Rosh (7:12): Rashi explains that Birkas ha'Zimun is said only with three. When there are only two, it is omitted. This is difficult, for the Halachah follows Rav Sheshes in Isurim, but we hold that every individual says Birkas ha'Zan! Also, we always discuss three Berachos of Birkas ha'Mazon! Also, we say (48b) that "u'Verachta" refers to Birkas ha'Zimun, and "Elokecha" refers to Birkas ha'Zan. This shows that Birkas ha'Zimun is not Birkas ha'Zan! Rather, the Gemara discusses one who interrupts for two. I.e. he interrupts for Birkas ha'Zimun, and resumes eating. Rav Nachman says that Birkas ha'Zimun is until Nevarech. Rav Sheshes says that it is until and including ha'Zan, for Nevarech is not a Berachah. Presumably, he interrupts eating for a Berachah, so it will be recognized that they are Mezamen with him. It is not because Birkas ha'Zan is Birkas ha'Zimun. Also individuals say Birkas ha'Zan! We thought that the Tana'im argue about this. Tana #1 holds that one interrupts until ha'Zan. Birkas ha'Mazon is two (other) Berachos for one who interrupted, and three for individuals (who did not interrupt). Tana #2 holds that one interrupts until Nevarech. Birkas ha'Mazon is three (other) Berachos for one who interrupted, and four including the Zimun. This is like the opinion that one who interrupted later resumes from where he stopped. (Ma'adanei Yom Tov - our Sugya holds like this.) According to the opinion that he returns to the beginning, Rav Sheshes would say that Beraisa #2 means that workers say three Berachos, and others say four.

Rosh (ibid.): I am unhappy with this Perush, for the Gemara asked Stam about Birkas ha'Zimun, and not particularly about one who interrupts to answer. Also, there is no reason to say that the Beraisos discuss this. The Rif's Perush is like most texts, which say 'two or three bless Birkas ha'Zimun (Bach's text -ha'Mazon).

Poskim

Shulchan Aruch (OC 200:2): One who ceases (eating to hear the Zimun) needs to interrupt only until "Baruch she'Achalnu mi'Shelo", and resumes eating without a Berachah Rishonah.

Beis Yosef (DH v'Da'as): It seems that the Rambam holds like the Rif, who rules like Rav Nachman. Therefore, we rule like them.

Rema: Some say that he must interrupt until ha'Zan Es ha'Kol. This is the custom.

Beis Yosef (DH u'Mafsik): The Tur holds like the Rif's Perush. However, the Rif rules like Rav Nachman, and the Tur rules like Rav Sheshes, for we follow him in Isurim. It seems that Tosfos and the Rosh agree. R. Yonah says that Bahag rules like Rav Sheshes. Why did the Tur says that the Zimun is until ha'Zan, i.e. Birkas ha'Zimun? This is Rashi's opinion. Tosfos and the Rosh rejected this! Rav Sheshes merely discusses how much one who interrupts must hear! The Tur merely cited Rashi entirely. He does not rule like him. He relies on what he wrote 'Birkas ha'Zimun is until...' There, he did not mention Rashi.

Taz (2): There are two matters of Zimun. One is what is said only when three ate, i.e. Nevarech. The other is how much one must hear before he leaves, i.e. Birkas ha'Zan. Rav Nachman holds that only the first is called Zimun. The Shulchan Aruch rules like him. The Rema brings Rav Sheshes's opinion, that there is also the second Zimun, which is until ha'Zan. They argue about one who interrupts to listen to two, and for one who needs to leave, i.e. how much he must hear first. This answers the Beis Yosef's question against the Tur.

Gra (DH v'Yesh): According to Tosfos, the Halachah follows Rav Sheshes in Isurim, and the Rif's proof is invalid (because even Rav Sheshes agrees that an individual says up to and including ha'Zan).

Mishnah Berurah (8): This opinion holds that even though ha'Zan is not totally from Birkas ha'Zimun, for even individuals say it, it pertains to Birkas ha'Zimun, for Nevarech itself is not a Berachah, for it lacks Shem u'Malchus (Hash-m's name and kingship). It alludes to what the Mezamen will bless, i.e. Birkas ha'Zan.

Mishnah Berurah (9): Just like one interrupts to make a Zimun with two others, three or four must interrupt to complete 10 for a Zimun with Hash-m's name. However, then all agree that they need to interrupt only until Baruch Elokeinu she'Achalnu mi'Shelo. If they ate together afterwards, they may make a Zimun. They did not lose their Zimun, for initially they joined only for Hash-m's name. Even if five interrupted for a Zimun with five others, they may join with five new people if they ate a k'Zayis together.

Kaf ha'Chayim (12): If six finished their meal and four did not, they must interrupt for the Zimun, since the majority finished. If five finished and five did not, but one of the latter five agrees to interrupt, the others need not agree, since only half truly finished.

Rema (ibid.): If he intended to eat bread again, even if he did not eat again, when he wants to bless, even though he did not eat afterwards, when he wants to bless he begins from Birkas ha'Zan, and all the more so if he ate again.

Magen Avraham (4): This is because from the beginning, he did not listen to Birkas ha'Zimun with intent to exempt himself from Birkas ha'Zimun.

Mishnah Berurah (7): If he did not intend to resume eating when he interrupted, and he reconsidered and ate, he must bless ha'Motzi, and this itself obligates him to wash his hands again.

Shulchan Aruch (193:1): If 10 ate together, they may not divide into groups of three, for they must be Mezamen with Hash-m's name. However, if in a large group people cannot hear the Mezamen, and they cannot separate into groups of 10 lest the Ba'al ha'Bayis hear and be upset, they may separate into groups of three and be Mezamen quietly, so the Ba'al ha'Bayis will not hear. This is better, for if they do not hear the Mezamen, they are not Yotzei the obligation of Birkas ha'Zimun.

Mishnah Berurah (17): The Acharonim disagree, for even though nowadays everyone blesses all Birkas ha'Mazon by himself quietly, still they must hear the Mevarech well and say with him word for word and answer Amen to every Berachah. In any case, surely they must do so until ha'Zan Es ha'Kol, which is Birkas ha'Zimun, like the Rema says. Therefore, even if they could hear Birkas ha'Zimun from the Mevarech, if they cannot hear until ha'Zan Es ha'Kol, it is better to divide into groups of three and be Mezamen without Hash-m's name, for then at least they are Yotzei the Zimun. At a large meal, someone with a loud voice should say Birkas ha'Zimun, so everyone will hear him.