either

First recorded before 900; Middle English; Old English ǣgther, contraction of ǣghwæther “each of two, both”; see ay¹, whether

When the pronoun either is the subject and comes immediately before the verb, the verb is singular: Either is good enough. Either grows well in this soil. When either is followed by a prepositional phrase with a plural object, there is a tendency to use a plural verb, but a singular verb is more common: Either of them is (or are) good enough. Either of the shrubs grows (or grow) well in this soil.
As an adjective either refers only to two of anything: either side of the river; using either hand. As a pronoun either sometimes occurs in reference to more than two ( either of the three children ), but any is more common in this construction ( any of the three children ). As a conjunction, either often introduces a series of more than two: The houses were finished with either cedar siding or stucco or brick. The pizza is topped with either anchovies, green peppers, or mushrooms.
Usage guides say that the verb used with subjects joined by the correlative conjunctions either … or (or neither … nor ) is singular or plural depending on the number of the noun or pronoun nearer the verb: Either the parents or the school determines the program. Either the school or the parents determine the program. Practice in this matter varies, however, and often the presence of one plural, no matter what its position, results in a plural verb: Either the parents or the school determine the program.
In carefully edited writing, these correlative conjunctions are usually placed so that what follows the first correlative is parallel to what follows the second: The damage was done by either the wind or vandals or either by the wind or by vandals (not done either by the wind or vandals). See also neither.

The pronunciations [ee-ther] /ˈi ðər/ and [nee-ther], /ˈni ðər/, with the vowel [ee] /i/ of see, are the usual ones in American English for the words either and neither. The pronunciations [ahy-ther] /ˈaɪ ðər/ and [nahy-ther], /ˈnaɪ ðər/, with the [ahy] /aɪ/ vowel of bite, occur occasionally for these words, chiefly in the speech of the educated and in the network standard English of radio and television. Both the [ee] /i/ and [ahy] /aɪ/ pronunciations existed in British English, and in the 19th century the [ahy] /aɪ/ came to predominate in standard British speech. In American English, therefore, it reflects a recent borrowing from British speech rather than a survival from the time of early settlement, influenced as well by the ei spelling, which is pronounced as [ahy] /aɪ/ in such words as height and stein.

Old English ǣgther, short for ǣghwæther each of two; related to Old Frisian ēider, Old High German ēogihweder; see each, whether

Either is followed by a singular verb in good usage: either is good; either of these books is useful. Care should be taken to avoid ambiguity when using either to mean both or each, as in the following sentence: a ship could be moored on either side of the channel. Agreement between the verb and its subject in either…or… constructions follows the pattern given for neither…nor…