Chapter 45: 1-3
Three or More People Reciting Grace 1. Three or more people who ate together are required to recite grace [in the manner described in Laws 5-7. This practice is referred to as] zimmun.
It is a mitzvah for them to recite grace over a cup [of wine or of another beverage of significance]. If possible, one must perform the mitzvah in the most distinguished manner, using a cup of wine. If it is impossible to use wine, one should use beer, mead, or brandy. The latter beverage must be chamar medina (literally, "the wine of the country") - i.e., if in this city, wine-grapes do not grow within a day's journey, hence, wine is expensive and one of these beverages is drunk it its place.
Certain opinions maintain that grace requires a cup of wine even when recited by a single individual. Those precise in the observance of the mitzvos do not hold a cup of wine when they recite grace alone. However, they place it on the table before them.
2. The cup of wine is poured and then the hands are washed [for mayim achronim; see Chapter 44, Law 1].
3. Once one has drunk from a cup of wine, what remains in the cup is considered as pagum (blemished) and is unfit to be used for grace. However, it may be "corrected," by pouring wine or a small amount of water from which no one has drunk, into it.
The wine must be poured into the cup for the sake of the blessing. Therefore, when correcting a cup of wine which is pagum, one should [first correct it,] then pour it back to the bottle, and then pour it into the cup with the intention of using it for the blessing.
