Q: Again on "Either"... does it mean "both"? I'm aware that the meaning of either can be:

*

*Any of two options

*Both the options

It depends from the context ...
This is the context, an official rule from the UK government:

When you arrive at the UK border you'll need to show either:

*

*a printed copy of the document attached to your confirmation email

*the document attached to the confirmation email on your phone


Now,
should I have with me BOTH the documents or just ONE ?
What is the (English grammar) rule here to identify if either means any or both?
[Update]
I read the answers in "can either mean both any and both" but I was still not sure because:

*

*the usage here is not "Either OF these options".

*The "both" meaning is an historical/old usage doesn't really matter.

*Google translator translates the phrase in Italian using "entrambi", that means BOTH, without any ambiguity!


A: Either does not mean both when it expresses an alternative. The required conjunction is or, never and.
Thus you need to provide one or the other; this is confirmed by the context, which is the requirement to show a QR code. Each form of the document has that, and you only need one form of the document to satisfy the requirement. The government's web page would certainly be improved by adding an or to the first alternative. However, to put and there would not make sense at all.
Either in "It could be on either side of the street" does express an alternative: it's assumed that of two possibilities, only one will apply. That doesn't mean "It could be on each side of the street," it means "It could on either this side of the street or that side." You can't say "It could be on this side and that side" when only one is possible.
Either in "There are shops down either side of the street" does not express an alternative: it actually has the same meaning as each side and does indicate that both sides of the sides of the street have shops. But that meaning is only possible because there is no either/or alternative.

A: The rule is poorly expressed according to conventional usage of either, which refers to two (or occasionally more) choices. One thing is clear from dictionary searches: either does not mean and.
The rule would be clearer if "... show either" were replaced by "... show either of". Alternatively, " ... show either A or B" (For brevity, I term the two choices A and B). Perhaps the writers felt that the bulleting of the choices relieved them from normal grammatical convention.
