neither … nor. The Columbia Guide to Standard American English. 1993
Neither is paired with nor as either is with or, and in those uses as conjunctions they pose usage problems of agreement. Usually they will take a singular verb if both parts of the structure are singular, as in Neither he nor his friend is ready, and if the first element is plural but the second element remains singular, the structure may still take a singular verb, as in Neither my friends nor my father is ready, although a plural is also possible. But if the second element is plural, the verb will almost always be plural: Neither my father nor his friends are ready. Agreement between neither/nor and the verb is frequently a matter of notional agreement: hence Standard English in all but its most Formal and Oratorical situations will usually accept either number of the verb.From In-depth Subject-Verb Agreement Study
Subjects joined by or, nor, either/or, neither/nor require further analysis.
Singular subjects joined by one of the above – use the singular verb. (Try the word it with the verb to see if you've made the right choice).
- The lettuce salad or the jello (it) is the least expensive. (it is)
- Neither a cup of tea nor a bottle of pop (it) helps at this point. (it helps)
Singular subjects mixed with plural subjects – choose the verb form which corresponds to the closer or closest subject.
- Neither Harry nor his friends see the danger. (they see)
- Neither his friends nor Harry sees the danger. (he sees)
See also this thread here (Post #4).