Use of either and neither, and their associated words or and nor, is complicated by disagreements about proper usage. Here’s a discussion of the words and the opinions about their appropriate use.
An “either . . . or” or “neither . . . nor” construction can include more than two elements, but some grammar guides advise writers to restrict the usage to reference to two choices: “I’m going to wear either blue or green” but not “I’m going to wear either blue, green, or red.” If you agree with that restriction (I don’t), delete either from the latter sentence — and it’s optional in the first one, for that matter. Omitting neither is not an option in the sentence “I’m going to wear neither blue nor green,” but the idea can also be rendered “I’m not going to wear blue or green.”
The proscription against using or, rather than nor, with neither (“I was neither here or there” instead of “I was neither here nor there”) is likewise not absolute, but nor is most common, and in this case I support the restriction.
The third divergence is about agreement with a verb. The more restrictive rule is that when either or neither is the subject, or part of the subject, of a sentence or a clause, it should be accompanied by a singular verb: “I don’t think either of the candidates is qualified.”
When two nouns or pronouns are framed by either and or, use a singular verb if the noun or pronoun closest to the verb is singular (“Either the boys or the girl is responsible”) and a plural verb if the closest noun or pronoun is plural (“Either the girl or the boys are responsible”) or both nouns or pronouns are plural (“Either the girls or the boys are responsible”).
However, it is common to see a plural verb with either or neither in reference to a sole plural noun or pronoun, as in “Neither of the parties are willing to compromise.” The more conservative choice is to write “Neither of the parties is willing to compromise,” and I adhere to that option.
Here are several other considerations: When constructing an “either . . . or” or a “neither . . . nor” statement, take care to place verbs appropriately. If one verb applies to both choices, place it before the “either . . . or” phrase: “She was going to leave either tomorrow or Saturday.” If a separate verb applies to each choice, either should precede the first verb, and or should come before the second one: “She was going to either leave tomorrow or wait until Saturday.”
Also, avoid using a “not . . . either” phrase, as in “They will not vote on either the amended proposal or the original one”; revise to a “neither . . . nor” construction: “They will vote on neither the amended proposal nor the original one.” Finally, a statement that two things are not true can also be rendered with a “never . . . nor” construction: “Their facility had never completed an evaluation nor ever met anyone representing the contracting agency.”