As for the other two expressions, they are in the same predicament, with an additional reason for the use of the plural. It contains the singular. The chief object of the present remarks has been less to explain details than to give due prominence to the following leading principles.
1. That either (or neither) is[[66]] essentially singular in number.
2. That it is, like any common noun, of the third person.
3. That it is pronominal where it is in apposition with another noun.
4. That when it is the first word of the proposition it determines the concord of the verb, unless its character of a noun of the singular number and third person be disguised by the prominence of some plural form, or some pronoun of the first or second person in the latter part of the term.
5. That in a simple disjunctive proposition (i.e., one where either does not occur) all nouns are subordinate to the first.
[§ 623]. I believe that the use of either is limited to real disjunctives; in other words, that we can say either a king or a queen always reigns in England, but that we cannot say either a sovereign or a supreme ruler always reigns in England.