Either the Mayor or her deputy [is/ are*] bound to come. [1]
What I say or what I think [is/ are*] no business of yours. [2]
Either the strikers or the bosses [has*/have] misunderstood the claim. [3]
:arrow:Either your brakes or your eyesight [is/ ?are] at fault. [4]
:arrow:Either your eyesight or your brakes [are/ ?is] at fault. [5]
Grammatical concord is clear when each member of the coordination has the same number: when they are both singular (as in [1] and [2]), the verb is singular; when they are both plural (as in [3]), the verb is plural. A dilemma arises when one member is singular and the other plural (as in [4] and [5]). Notionally, or is disjunctive, so that each member is separately related to the verb rather than the two members being considered one unit, as when the coordinator is additive and. Since the dilemma is not clearly resolvable by the principles of grammatical concord or notional concord, recourse is generally had to the principle of proximity: whichever phrase comes last determines the number of the verb, as in [4] and [5].