In a thread about marriages, @robby linked to a 1970s Texas law that gave a long list of who may perform marriages. It ended with this line:
YOU, ОR EITHER OF YOU, ARE HEREBY AUTHORIZED TO JOIN In the Holy Union of Matrimony
Why “either”? Wouldn’t “any” be a more natural usage? The law has its own vocabulary for the usual arcane reasons, I know, so this probably has some ancient roots. And the phrasing is ambiguous in another way. Who is “you”, the couple being married or the officiants listed?
I read “either” in the sense of “any and all of you” and it was probably inserted into the law to eliminate ambiguity (in modern English, “you” is used as both a singular and a plural).
From the context, I read it to mean that the law authorizes all those who are of one of the enumerated classes of people to join in matrimony (I.E., to marry in the sense of officiating at the weddings of) the brides and grooms.
That seemed the mostly likely explanation.
It sounds like you haven’t seen this usage of “either” before. I can’t remember ever encountering it, in law or elsewhere. I wonder where it came from, and if Texas law has always used it.
Apparently “either” in reference to more than two things is an old meaning, although it is not completely extinct as shown below. It is of course common in legal terminology to use archaic language.
The OED gives several meanings of “either” where it applies to more than two things:
I.4 Each of more than two. Obsolete. 1565–1893
(example citation) 1867 Just above the feet, at either of the three corners, is an exquisite..female bust.
II.6 Any one of more than two. Now nonstandard. 1616-
(example citation) 2006 The lack of statistically significant context effect in either of the three studies is explained against the background of Finn’s..‘transfer feature theory’.
It isn’t uncommon in legal documents. It refers to “joint or several” rights and liabilities. They apply to people together as a group, or to each person separately on their own.