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Could you please clarify when a kos (cup of wine) should be used for Zimun?
Maran in Shulhan Aruch (O.H. 182:1) quotes three different opinions as to when a kos is required for Birkat HaMazon:
One opinion holds that anytime one recites Birkat HaMazon one must recite the it with a kos. This entails holding a full cup of wine during Birkat HaMazon, reciting Bore P’ri HaGefen at the end of Birkat HaMazon and drinking the cup of wine. According to this view if two people ate together, since there is no Zimun, each person must have their own cup of wine.
The second opinion is that there is only a requirement of a kos when there is a Zimun, i.e., three or more men reciting Birkat HaMazon together. Only the leader needs to hold the cup of wine.
The third opinion is that there is no obligation to have a kos even if one is Birkat HaMazon with a Zimun.
The Mishnah Berurah (182:4) writes that although the common custom is in accordance with the third opinion, that even if one recites Birkat HaMazon with a Zimun they are not obligated to have a kos, if one wishes to fulfill the Mitzvah in the optimal manner, they should arrange to recite Birkat HaMazon with a kos. However, if one is reciting Birkat HaMazon without a Zimun then the Minhag is not to use a kos.
Do Sepharadim recite the first Beracha of Birkat HaMazon out loud?
There is a difference between Ashkenazim and Sephardim. The custom of Sephardim is in accordance with Maran (O.H. 183:7) who writes that everyone should recite Birkat HaMazon quietly word by word together with the leader, including even the end of each Beracha. The Rama adds that everyone should hurry to finish the Beracha before the Mezamen so that they can answer Amen, which is the basis for the Ashkenazi practice.
Yalkut Yosef (Dinei Zimun 4-5) explains that according to Maran, it is inappropriate for the leader to say the end of the Beracha out loud and cause the others to answer Amen, since this would constitute a Hefsek (interruption) in the middle of one’s Birkat HaMazon. According to Maran, Birkat HaMazon is similar to Shmone Esre and one is not permitted to interrupt in the middle of Birkat HaMazon even to answer Amen. However, if a Sepharadi is leading a Zimun for Ashkenazim, then the Yalkut Yosef writes that he should end the Beracha out loud, so that the Ashkenazim can answer Amen in accordance with the ruling of the Rama.
What is the proper way to hold the Kos Shel Beracha (cup of wine used e.g., for Kiddush, Havdalah or Birkat HaMazon)? I have seen some people hold the cup from the bottom while I have seen others hold it from the middle. Which is correct?
The Mishnah Berurah (183:14-16) writes that one should hold the cup in one’s right hand, and lift the cup at least a tefah (approximately 3-4 inches) off the table. One should not support the cup with one’s left hand. Mishnah Berurah quotes the Shela who writes that according to Kabbalah the proper way to hold the cup is on the palm of the right hand with the fingers extending upwards around the cup. The source for this is the Zohar. However, the Magen Avraham (183:6) writes that the wording of the Zohar is inconclusive. It can also be interpreted to refer to wrapping one’s fingers around the middle of the kos. The Bach (183:6) maintains that one may hold the kos in the middle. Indeed, there were great tzadikim who specifically held the kos in this manner (see VaYa’as Avraham p. 445). We see that both customs have a valid basis in Halacha and everyone should follow his Mesorah from his parents and ancestors.