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A Jew can fulfill the obligation of another person based on the mechanism of when a person listens to someone else recite a certain text it is like they said it themselves and they fulfill their obligation. That mechanism is called Shomea KeOneh.
- Even though there is always a mitzvah to answer the bracha of a fellow Jew, there is an added reason why a person should answer Amen when one is listening to a bracha, which is in order to fulfill one's obligation. 
- In order to fulfill one’s obligation the listener must hear the whole bracha starting from the word Baruch.  If one did miss part of the bracha some say that one can fill in that part by saying those specific words. 
- Some say that if one answered Amen to the bracha even if one missed hearing part of it one fulfilled his obligation, therefore, after the fact one shouldn't recite the bracha again.
- If one was talking while listening to a bracha one has not fulfilled one’s obligation. 
- According to the Moroccan custom, even one who is fulfilling a mitzvah via Shomea KeOneh should answer Baruch Hu Uvaruch Shemo to the bracha being said. 
- The one making the bracha must have intent to fulfill the obligation of the one listening and the one listening to the bracha must have intent to fulfill his obligation through listening. Otherwise one doesn't fulfill one's obligation. Others argue that after the fact one fulfilled one's obligation.
- Listening to someone's bracha with Shomea Konah isn't permitted in front of ervah since one couldn't possibly have said it oneself.
Brov Am Hadrat Melech
- It is preferable to have multiple listen to one bracha rather than have everyone say it themselves because of the principle of Brov Am Hadrat Melech, Hashem is honored with a greater multitude of people. This concept trumps the idea of Mtizvah Bo Yoter Mebeshelucho. Although it is possible for each individual to recite their own bracha and everyone have negative kavana and then to recite it themselves, it is better to say the bracha together.
Brachot on Food
- For all brachot for food, either Bracha Rishona or Bracha Achrona, one may not fulfill one’s obligation by listening to someone say the bracha unless that person is also going to eat or has eaten the proper Shuir.  The notable exceptions to this are Hagefen for Kiddush and Hamotzei for matzah on the first night of Pesach; for those, it is permissible to fulfill someone else's obligation even if one is not going to eat.
- The original establishment of the rabbis was to make Bracha Rishona together (meaning, one person saying it out loud and everyone else fulfilling the obligation by listening). 
- Some poskim hold that we would not employ Shomea KeOneh for a Bracha Rishona except for HaMotzei on bread and HaGafen on wine.  Additionally, since people are not experts in having intent to fulfill the obligation of others and those listening having intent to fulfill their own obligation, the minhag is that each person make the Bracha Rishona for themselves. 
- It is preferable to say a bracha together and this is based on the principle of BeRov Am Hadarat Melech, meaning, that it is more respectable to serve Hashem in multitudes. 
- In order that one should fulfill someone else's obligation of a Bracha Rishona with Shomea KeOneh everyone must sit together at the same table.  However, after the fact even those who did not eat at the same table can fulfill the obligation by listening to someone else who is making the bracha if the one making the bracha has intent to fulfill the obligation of others and the one listening has intent to fulfill his own obligation by listening. 
- If someone heard a Bracha Rishona and had intent to fulfill one's obligation through the agency of Shomea Keonah, but then changes one's mind and does not want to eat, if he does not eat, he is considered to have made a Bracha Levatala, therefore one should eat anyway. 
- The original establishment of the rabbis was for each person to make Bracha Achrona to themself, except Birkat HaMazon which is supposed to be said together (meaning, one person saying it out loud and everyone else fulfilling the obligation by listening).  Part of the original enactment of why it was preferable that a person fulfill the obligation through listening (to someone else), was for the case in which the person does not know the text of the Bracha Achrona. 
- Nowadays, since Bracha Achrona is often disregarded and forgotten, it is considered preferable that the Bracha Achrona be made out loud and everyone listenand fulfilltheir obligation that way. It is proper that everyone say the bracha along with the one saying it out loud word by word. 
- Nowadays, because it is difficult to have proper intent for a long time the minhag is for each person to say Birkat HaMazon silently to oneself while the Mezamen reads it out loud. 
- In order that one should fulfill the obligation of the other by saying Birkat HaMazon together, everyone must sit together at the same table.  However, after the fact even those who did not eat at the same table can fulfill the obligation together (meaning, one person saying it out loud and everyone else fulfilling the obligation by listening). 
Brachot on Mitzvot
- The Yerushalmi (Brachot 3:3) establishes that for everything one can fulfill the obligation of someone else with the exception of Shemona Esrei, Shema, and Birkat Hamazon.
- Someone who is obligated in a mitzvah can fulfill the obligation of someone else who is also obligated in that mitzvah. Someone who is exempt can't exempt someone who is obligated in the mitzvah.
- Someone who is obligated in a mitzvah on a rabbinic level can't fulfill the obligation of someone who is obligated on a biblical level.
- Someone who is obligated in a rabbinic mitzvah on a rabbinic level can fulfill the obligation of someone who is completely obligated in that rabbinic mitzvah.
- Some say that if everyone listening to the bracha knows how to recite the bracha themselves they shouldn't use yatzah motzei.
- If a person already fulfilled his obligation he can nonetheless fulfill the obligation of others based on the concept that all Jews are responsible for one another. That concept is called Yatzah Motzei. This concept works for biblical and rabbinic mitzvot.
- Regarding Brachot HaMitzvah, even if one has already fulfilled one’s obligation one can still fulfill the obligation of others because of the principle of Aravim Zeh BaZeh, the responsibility for our fellow Jew. 
- For Birchat Hanehenin one may not exempt someone else without eating themselves because it isn't obligatory to eat and so there's no obligation for one Jew to exempt another Jew unless he is also going to eat. Some exceptions include Kiddush Friday night, Kiddush Shabbat day, and Hamotzei of Motzei Matzah.
- For a Bracha of Shevach such as Birchot Hashachar, Baruch She'amar, and Yishtabach there is a dispute whether one fulfill someone else's obligation even though one already fulfilled one's obligation. 
- According to Sephardim one may not fulfill the obligation of someone else in asher yatzer unless one is personally obligated.
- If one already fulfilled Kiddush or didn't yet fulfill it but isn't going to fulfill it with this Kiddush he can still recite it for another person even if one doesn't eat it there his meal. However, he needs to give the cup to someone else to drink a Reviyit or a majority of it. Yet, if the person who is listening knows how to make Kiddush he should make it himself.
- If one didn't yet fulfill his mitzvah but is going to later on he can be motzei others in their mitzvah.
If One Isn't Currently Obligated
- Someone who isn't obligated in Megillah that day can't fulfill the obligation of someone who is obligated that day.
- In an extenuating circumstance it is permitted to recite kiddush for someone who accepted Shabbat even though the one didn’t accept Shabbat and plans to do melacha afterwards. One may not use this leniency on a regular basis.
- Some held that if a person forgot to count the sefira one night it is permitted to recite the bracha for someone else who did count every night, but others disagree.
- See Yom Tov Sheni for an Israeli fulfilling the obligation of an American in Kiddush on the second day of Yom Tov.
- See Yom Tov Sheni for an Israeli fulfilling the obligation of an American congregation on the second day of Yom Tov.
- If someone isn't fasting they can't be the Shaliach Tzibur for a congregation of those who are fasting.
- One who hears kiddush must understand the words of the one making the blessing. 
- If one is saying Shmoneh Esrei and one hears the Shaliach Tzibbur reach Kedusha one should pause and listen but one should not answer with the congregation. By listening to the Shaliach Tzibbur one fulfills one's obligation through Shomea KeOneh. One does not need to raise one's feet when listening to Kedusha. 
- Mishna Brurah 213:17, Mishna Brurah 8:15, See Vezot HaBracha pg 362 regarding whether answering Amen makes it like the person listening said the entire Bracha.
- Mishna Brurah 213:19, Shaar HaTziyun there mentions that if one did not hear some words, as long as they are not the crucial ones, one is still considered to have fulfilled the obligation by listening. (Shulchan Aruch 214:1 and Mishna Brurah 314:4 specify Baruch, either Hashem or Elokenu, Melech, and HaOlam and the conclusion as the crucial words).
- Vezot HaBracha (pg 362) quoting Halichot Shlomo (Klali Brachot 11).
- Halacha Brurah 213:14 based on Rabbenu Yonah Brachot 35a and others
- Mishna Brurah 167:45. Vezot HaBracha pg 361 writes that it is possible that the Chazon Ish 141:7 who’s explanation of Shomea KeOneh is that the listener joins the bracha by actively listening would disagree.
- Kitzur Shulchan Aruch of Rav Rephael Baruch Toledano, Volume 1, Page 111
- Mishna Brurah 213:5, 18
- Halacha Brurah 213 fnt. 10 writes that we're concerned for the opinion that the need for kavana in shomea k'oneh depends on whether mitzvot need kavana in general (Bet Yosef 213:3 based on Rosh Hashana 29a). If so, according to the rishonim that mitzvot don't need kavana or particularly brachot derabbanan don't need kavana one would fulfill the bracha he heard even if he and the one speaking didn't have kavana. Shulchan Aruch Harav 213:4 is also concerned for this opinion as well. See further Chazon Ovadia Brachot 353 and Birkat Hashem 1:4:1.
- Mishna Brurah 75:29
- Mishna Brurah 213:3
- Tosefta Brachot 6:20 states that ten people who are fulfilling a mitzvah together should have one person recite the bracha on behalf of all of them. Rosh Pesachim 1:10 cites this. Maharashdam OC 1 writes that although it is possible to have negative kavana and a person should have to recite their own bracha it isn't a proper practice since it increases the amount of brachot made and it is preferable to minimize those. See also Mabit 1:117. Shulchan Aruch Harav 59:4 writes that brov am hadrat melech applies to birchot hashachar. Vayikra Avraham YD 4 writes that agrees and proves it from Shulchan Aruch O.C. 8, 66, 296, 298, 432, 489, Y.D. 265, E.H. 62. [Several sources indicate that it is fine to split it up: S”A O.C. 6:5, 8:5, 296:7, 489:1, and Magen Avraham 619:3. Several sources indicate that you should not split up: 298:14, 432:2, YD 265:5, EH 62:3.]
- Mishna Brurah 213:14
- Shulchan Aruch 167:20. Magen Avraham 167:41 explains that in general one cannot fulfill someone else's obligation for a food bracha, unless one is going to eat as well, because unlike for matzah and Kiddush, there is no halachic obligation to eat.
- Shulchan Aruch 213:1
- Rama 213:1
- Mishna Brurah 213:12
- Mishna Brurah 213:3
- Shulchan Aruch 213:1
- Mishna Brurah 213:5
- Sh”t Yabia Omer O”C 8:24
- Shulchan Aruch 213:1
- Mishna Brurah 213:9
- Mishna Brurah 213:9
- Shulchan Aruch 183:7 writes that it is proper that each person say Birkat HaMazon silently to oneself along with the bracha that the Mezamen is saying out loud. Mishna Brurah 187:27 adds that the original enactment was for everyone to listen to the Mezamen make Birkat HaMazon out loud, however, nowadays that it is difficult to have proper intent for that period of time, everyone should read along silently.
- Shulchan Aruch 213:1
- Mishna Brurah 213:5
- Rosh Hashana 29a, Brachot 20b, Rambam Brachot 1:11
- Brachot 20b. Rashi Brachot 48a s.v. ad writes that once a person is obligated in a mitzvah rabbinically he can fulfill the obligation of someone who is obligated biblically. That point is disputed by the Bahag cited by Rashi, Tosfot 48a s.v. ad, Rosh Brachot 7:21, Rambam Brachot 1:11, Rabbenu Yonah Brachot 35b umha, and Rashba 48a s.v. yahavu. According to Tosfot, the fact that you need to eat in order to say Birkat Hamazon isn't a perfect example of something you need to be obligated in in order to fulfill others since biblically you could say Birkat Hamazon without eating at all based on Yatzah Motzei. However, the Mordechai (Megillah n. 798) implies that it is permitted to do so and his proof is that we recite Kiddush during Tosefet Shabbat and fulfill Kiddush for Shabbat biblically.
- Tosfot Megillah 19b writes that a child which is obligated in megillah on a rabbinic level can't fulfill the obligation of an adult in megillah which is only a rabbinic mitzvah because a double derabbanan can't fulfill the obligation of a single derabbanan. Rabbi Akiva Eiger Brachot 20b s.v. elah points out that a child who ate a kezayit can fulfill the obligation of an adult who ate a kezayit even though the child is a double derabbanan and the adult is a simple derabbanan since birkat hamazon is different and even if you didn't eat at all biblically you can fulfill the obligation of others.
- Shulchan Aruch 273:4 writes that one should only say kiddush for others if they already fulfilled their obligation if the others don't know how to say kiddush. Pri Chadash 273:4 disagrees but adds that if those listening answer amen it works even according to Shulchan Aruch. Biur Halacha 273:4 s.v. vhu writes that it is purely a mitzvah that since the one listening knows how to say it and they are going to fulfill their obligation they should be the one to say it but certainly after the fact they fulfill their obligation.
- Rosh Hashana 29a, Shulchan Aruch 273:4
- Even though the Turei Even Rosh Hashana 29b holds that it is only effective on rabbinic mitzvot, we don't accept his opinion (Minchat Shlomo 1:3). Also, even though the Tzlach Brachot 48a holds that it is only effective on biblical mitzvot, we don't accept his opinion (Meor Yisrael Brachot 48a s.v. maan). The Rif Pesachim 27a and Rosh Pesachim 10:36 write that it is possible to have yatzah motzei for maror even though maror today is rabbinic. Shulchan Aruch 484:1 codifies the Rif and Rosh. Meor Yisrael cites the Birkei Yosef 124:3, Imrei Binah Shabbat n. 11, and Chatan Sofer 129 who assume not like the Tzlach. Ritva Hilchot Brachot 2:24 seems to be a proof for the Tzlach.
- Zera Avraham 1:12 writes that there's no such thing as arvut on mitzvot derabbanan since we only entered the convert of responsibility for all Jews on the mitzvot we had at the time of Har Grizim and Avel and not mitzvot derabbanan. Ginat Veradim 1:13 assumed otherwise. The Birkei Yosef 124:3 proves that it applies to derabbanan's as well from Rashi Rosh Hashana 29a who says that avrut should have applied to all brachot hanehenin but it doesn't since one isn't obligated to eat. The Ran and Tashbetz 3:79 agree. Since birchat hanehenin are derabbanan, it is clear that arvut applies to derabbanan's. He repeats this in Chaim Shaal 1:75 and adds that Tosfot Brachot 48a also holds that arvut applies to derabbanan mitzvot.
- Mishna Brurah 213:14
- Rosh Hashana 29b, Shulchan Aruch 167:20. Rabbi Akiva Eiger Pesachim 106a s.v. hava writes that from Tosfot it is evident that even for the Shabbat day Kiddush it is possible to have yatzah motzei even if the one making the bracha doesn't drink. That is also the opinion of Shulchan Aruch 167:20 and Aruch Hashulchan 167:32.
- The Ritva (Rosh Hashana 29a s.v. tani) writes just like there is a concept of yatzah motzei for birchot hamitzvah there is also one for birchot hashevach. Hagahot Ashuri (Rosh Hashana 3:13) agrees and applies it to Birchat Hashachar and Baruch She'amar. The Meiri (Rosh Hashana 29a s.v. brachot), however, argues that one may not fulfill the obligation of someone else for birchot hashevach just like for tefillah. He concludes that for Yishtabach if one already fulfill his obligation he can't repeat it for the congregation.
- Mishna Brurah 6:10 rules that for Birchot Hashachar one can fulfill someone else's obligation even though one already fulfilled it.
- Mishna Brurah 59:21 extends the concept of Yatzah Motzei to Birchot Kriyat Shema. Halacha Brurah 59:14 writes that one shouldn't fulfill the obligation of someone else if they know how to say it themselves but after the fact it is effective.
- Yalkut Yosef 6:11. See also Yabia Omer OC 9:3:2.
- Shulchan Aruch 273:4 writes that only if the person doesn't know how to recite kiddush can someone else make kiddush for them even though he isn't going to fulfill his obligation. The Mishna Brurah 273:20 disagrees and holds that it is only preferable to let the person hearing Kiddush and knows how to recite it to do it himself but after the fact certainly it is effective. Rav Ovadia Yosef in Kol Sinai Tevet 5724) agrees after the fact.
- Rif Pesachim 27a, Tur and Shulchan Aruch O.C. 484:1. The Rif says that you can make kiddush and the whole seder for many families either by first having the seder at home and then in everyone else's home or the opposite. The Tur 484:1 and Shulchan Aruch codify this as the halacha. Mishna Brurah 484:9 explains that even if you don't have kavana to be yotzei at the first person's home you can still be motzei the others but then one can't eat there since one didn't yet fulfill kiddush.
- Yerushalmi 2:3 writes that someone from outside Yerushalayim can’t fulfill the obligation of someone who lives in Yerushalayim on the 15th. The gemara has a question about the opposite case. Tosfot Yevamot 14a s.v. ki holds that someone obligated one day can’t fulfill the obligation of someone obligated another day. Rosh Yevamot 1:9 and Ran 2b s.v. vyesh agree. However, Rashi Megillah 2a s.v. elah that someone from a city can read the megillah before the 14th for the small villages.
- Rav Shlomo Zalman Auerbach in Minchat Shlomo 1:3 writes that it is permitted on a temporary basis for someone to recite kiddush for someone else who accepted Shabbat and for the one making kiddush not to accept Shabbat. He explains that biblically one can fulfill his obligation even if one isn’t obligated in that mitzvah right now (relevant to someone who lives in Yerushalayim fulfilling the obligation of someone outside Yerushalayim on the 14th of Adar). Also, according to the Rambam it is sufficient to say Kiddush close to Shabbat even though it isn’t Shabbat right now. However, since Rabbi Akiva Eiger isn’t sure about it and there are reasons to question whether it is possible to say kiddush without accepting Shabbat, one shouldn’t rely on this leniency consistently or create a minhag with it. Rav Yitzchak Yosef (Motzei Shabbat Shemot 5779 min 43) and Tzitz Eliezer 14:25 were lenient.
- Har Tzvi 2:75 quotes that the Bet Halevi once forgot to count sefirat haomer one night and when he was shaliach tzibur he asked someone to listen to his bracha with intent to fulfill his obligation. The Har Tzvi points out that the Knesset Hagedola and Pri Chadash Pri Chadash 489:8 argue that it is impossible for someone who didn’t count one night to fulfill the obligation of someone who counted every night since the one who forgot once is like he isn’t obligated in the mitzvah any more. They compare it to the case of someone who lives in Yerushalayim fulfilling the obligation of megillah for someone outside Yerushalayim on the 14th of Adar. Har Tzvi defends the Bet Halevi. Chashukei Chemed Niddah 73a is strict.
- The Tur 566:4 quotes Rav Natan Goan who held that if someone isn't fasting he can't act as the Shaliach Tzibur but the Tur disagrees since he isn't lying when he says in Anenu it is a fast day even though he's not fasting. The Shulchan Aruch 566:4 codifies Rabbi Natan Goan. Mishna Brurah 566:18 agrees.
- Hacham Ovadia (Hazon Ovadia, Volume 2, Page 18) writes that a Sepharadi who does not understand kiddush in an Ashkenazic or Yemenite accent can not fulfill the obligation via shomea ke'one in such a way.
- Shulchan Aruch 104:7. This is based on Rashi Sukkah 38b s.v. Hu who says that Shomea KeOneh makes it possible to fulfill one's obligation by listen and not answering. However, Tosfot s.v. Shamah argues that perhaps listening is considered an interruption in one's Shmoneh Esrei.
- Yabia Omer 6:16