Gemara

(Mishnah): If one was Motzi enough [undiluted] wine to mix [with water] to comprise a cup, he is liable.

(Beraisa): This refers to the amount of wine to mix a nice cup.

This is the Kos of Berachah [over which we say Birkas ha'Mazon].

93a (Rav Yehudah): If two did a Melachah, and one of them could have done it himself, but the other could not, all agree that he is liable.

(Rav Chisda): The one who could have done it is liable. The other did not do anything!

93b - Support (Ravina - Beraisa): If a Kohen did Avodah with his right hand and the left hand helped, the Avodah is Kosher.

This is because [even though Avodah must be with the right hand], help is insignificant.

Berachos 51a (R. Zeira): Ten things were said about the Kos Shel Berachah:

It must be washed and rinsed;

It must be Chai (whole; alternatively the wine must be undiluted) and full;

It requires Itur (crowning) and Ituf (covering the head) ;

One receives it with both hands, and puts it in his right hand;

He lifts it a Tefach off the ground, and looks at it.

Some say, he sends it for a gift to his wife (to taste from it).

(R. Yochanan): We require four of these -- it must be washed, rinsed, Chai, and full.

(Beraisa): "Washing" is from the inside. "Rinsing" is from the outside.

(R. Yochanan): Anyone who blesses on a full cup receives an unlimited inheritance -- "u'Malei Birkas Hash-m Yam v'Darom Yerashah."

To fulfill "Itur," Rav Yehudah would crown it with Talmidim. Rav Chisda would surround it with other cups.

To fulfill "Ituf," Rav Papa would cloak himself with his Talis; Rav Asi would cover his head with a turban.

(Beraisa): One receives it with both hands.

(R. Chanina bar Papa): We learn from "Se'u Yedeichem Kodesh u'Varchu Es Hash-m."

(Beraisa): He puts it in his right hand.

(R. Yochanan): Early Chachamim were unsure whether or not the left hand may help support it.

(Rav Ashi): Since they were unsure, we are stringent.

(Beraisa): He lifts it a Tefach off the ground.

(Rav Acha bar Chanina): We learn from "Kos Yeshu'os Esa uv'Shem Hash-m Ekra."

He looks at it, so that he will not take his mind off it.

He sends it for a gift to his wife, in order that she will be blessed.

Rishonim

Rosh (Berachos 7:35): R. Yochanan said that we require only four of these. He excludes only Itur and Ituf. He agrees that he puts it in his right hand, for he said that early Chachamim were unsure whether the left hand may help. Presumably, he also agrees that one receives it with both hands and lifts it, for we expound these from verses. It is reasonable to look at it, lest his mind be diverted. Rav Nachman used to send it to his wife. Also all of the others we are concerned for nowadays, even Ituf (we do not bless with an uncovered head). We lack only Itur.

Tosfos (51a DH Ein): It seems that R. Yochanan agrees that one receives it with both hands, and puts it in his right hand. R. Yochanan taught that early Chachamim were unsure whether the left hand may help! This shows that the custom was to hold it with the right hand. However, since the Amora'im fulfilled Itur and Ituf, this connotes that they were stringent about all of them. Therefore, we should be stringent about the Kos Shel Berachah.

Gra (OC 183:4 DH Mekablo, based on Damesek Eliezer): [Tosfos explains that R. Yochanan requires four laws regarding the cup itself. He does not exclude receiving it with both hands, and putting it in his right hand, which are not laws of the cup itself.] I [the Gra] explain that only four laws are Me'akev. This is why Rav Ashi said 'since early Chachamim were unsure, we are stringent.' Seemingly, also they needed to be stringent due to Safek! Rather, they held that all 10 laws are Me'akev. They could not be stringent not to help with the left, lest this come to a leniency, lest he need to put the cup down. It is Me'akev to hold it! We hold that this is only l'Chatchilah, therefore we can be stringent about helping with the left hand. The Rosh, and Tur in the name of Tosfos, say that R. Yochanan excludes only Itur and Ituf. Also the Mechaber and Rema omitted Itur and Ituf. What I said is primary. Also Ma'adanei Yom Tov (90) says so.

Tur (OC 183): The Rambam brings only the four that R. Yochanan mentioned, but Tosfos says that R. Yochanan excludes only Itur and Ituf.

Rashi (76b DH Kos): Chachamim commanded to beautify the Kos Shel Berachah. It requires Itur, Ituf, washing rinsing, Chai and full.

Tosfos (51a DH Shetifah mib'Chutz): The Ri requires washing the inside and rinsing the outside only if it is not nice, and has remnants from previous cups. If it is nice without washing and rinsing, this is fine. We require only that the Kos Shel Berachah be nice.

R. Yonah (Berachos 37b DH Anan): The custom is rinse and wash it [in any case], even though the Ri says so only if it has remnants from previous cups, However, if one did not drink from it during the meal, and now they take it out clean for Birkas ha'Mazon, surely one need not rinse and wash it.

Poskim

Shulchan Aruch (OC 183:1): The Kos Shel Berachah must be wash inside and rinsed outside.

Kaf ha'Chayim (1): The Shulchan Aruch rules like the Tur and Tosfos. (He brings the other laws in the coming Sa'ifim.) Machazik Berachah, Alfasi Zuta, R. Yonah and the Rashba say that only the four matters are Me'akev. The Zohar mentions the 10 matters.

Kaf ha'Chayim (2): Tosfos and the Rosh say that the custom is to properly cover the head. One should bless with a hat, and not just a Kipah, even when blessing alone.

Shulchan Aruch (ibid.): If it is clean and there are no remnants from previous cups, it need not be washed and rinsed.

Magen Avraham (1): 'Remnants from cups' refers to wine in which one soaked bread. Likewise, it suffices if one cleaned it with a cloth.

Gra: This is like Tosfos, and Rashi in Shabbos [who says that it must be rinsed and washed so it will be nice].

Mishnah Berurah (3): It is good to rinse it even if there are no leftover crumbs, unless it is clean and clear.

Kaf ha'Chayim (4): The Zohar connotes that one must wash and rinse it even if it is clean.

Shulchan Aruch (4): One picks up the cup with both hands, and when he begins to bless puts it in his right hand. He does not help with his left hand.

Beis Yosef (DH u'Mah she'Chasav Hilkach): Since it says 'one receives it with both hands, and puts it in his right hand', we must say that he takes it with both hands before beginning to bless.

Taz (2): This is to show dearness of taking the cup. He desires to take it with all his strength. Afterwards, he holds it in one hand, lest it seem that it is a burden upon him. To honor the cup, he makes the right hand primary, for it is more important

Kaf ha'Chayim (19): The Zohar says that someone else gives the cup to the Mevarech with two hands, and he receives it with two hands.

Rema: This means that the left hand may not touch the cup. He may put his left hand under the right hand to help.

Beis Yosef (DH u'Mah she'Chasav Anan): Shibolei ha'Leket says that R. Binyamin forbids holding it totally in the left hand, but the left hand may be under the right hand, for we hold that helping is insignificant. This is difficult. The Gemara said that we are stringent! He must hold that the Gemara asked about holding the cup also with the left hand, but if the left hand is under the right hand and helps, there is no problem.

Taz (3): The Tur says that the Gemara was unsure whether one may help the right hand with the left, i.e. even if the left hand does not touch the cup, rather, it only supports the right hand. If the left hand supports the cup, it should have said 'Somcho' (masculine). The Beis Yosef says that it may help support the right hand. This is astounding, for we conclude that we are stringent! He must explain that the question was to support the cup, but if it the left hand does not touch the cup, it is surely permitted, for help is insignificant. Based on this, the Rema distinguishes between whether or not he touches the cup. Why did he ignore the Tur, whom we always rely on, to follow a less renowned Posek (Shibolei ha'Leket)? I say that one should not rely on this leniency. Any support is forbidden. Even though [normally] help is insignificant, here Chachamim forbade.

Magen Avraham (6): The Bach says that Shibolei ha'Leket permits when the left hand touches the cup. This is wrong. He holds like the Rema says, that it may support only the hand. I support Shibolei ha'Leket from Shabbos 93b. Avodah is Kosher even if the left hand helped the right hand. The Tur explains that the question was whether the left may help the. I say that he found it difficult to say that we asked whether also the left hand may hold the cup, for we said that one takes the cup with two hands and holds it in the right hand! If so, we asked about supporting the right hand, and we conclude that we are stringent. Therefore, one should be stringent, unless there is a need [to be lenient], like Tosfos says about a big cup. The Shlah brings from Mekubalim to put the cup in the palm, and the fingers are erect around the cup. The Zohar connotes like this, but one could explain differently, that he surrounds it with his fingers the way one [normally] holds [things]. One does not take the cup with gloves on. He removes them first.

Mishnah Berurah (13): One may not even hold the cup in the middle with his right hand, and below with the left hand.

Mishnah Berurah (15): One should be stringent about using the left hand to support the right hand, unless it is needed.