Q: Why do people use “either” when they mean both? For instance, I’ve heard traffic reporters say the GW Bridge is “clogged in either direction.” Doesn’t “either” imply an alternate – either I do this or that, not both?
A: The word “either” serves as several different parts of speech; it can be an adjective, an adverb, a pronoun, or a conjunction. Only when it’s used as a conjunction does it require “or.”
In the use you mention, “clogged in either direction,” it’s correctly used as an adjective. In this sense, its meaning is “each” or “one and/or the other.”
As an adverb, its meaning is “likewise” or “also” or “as well,” as in “She isn’t going and I’m not either.”
As a pronoun, its meaning is “the one or the other,” as in “Either will do.”
As a conjunction, it’s used before two or more items or clauses linked by “or,” as in, “Either this or that,” or “After dinner, either he reads or he returns phone calls.”
Notice that I said “two or more” above. When used as a conjunction, “either” implies one of two or more elements.
However, if it’s an adjective (meaning “one and/or the other”) or a pronoun (meaning “the one or the other”), then “either” implies one of two only.
Here’s how The American Heritage Dictionary of the English Language (4th ed.) explains this:
“The traditional rule holds that either should be used only to refer to one of two items and that any is required when more than two items are involved: Any (not either) of the three opposition candidates still in the race would make a better president than the incumbent. But reputable writers have often violated this rule, and in any case it applies only to the use of either as a pronoun or an adjective. When either is used as a conjunction, no paraphrase with any is available, and so either is unexceptionable even when it applies to more than two clauses: Either the union will make a counteroffer or the original bid will be refused by the board or the deal will go ahead as scheduled.“
Merriam-Webster’s Collegiate Dictionary (11th ed.) agrees. When “either” is a conjunction, M-W says, it’s used “before two or more coordinate words, phrases, or clauses joined usu. by or to indicate that what immediately follows is the first of two or more alternatives.”
I hope this helps.