Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
AGREEMENT OF SUBJECTS AND VERBS 1: CLAUSES BEGINNING WITH WHAT USED AS SUBJECTS
Notional agreement seems to govern the number of the verb following a what clause. Consider these Standard examples: What is her name? What are their names? Here name and names govern whether what is to be singular or plural. But when the what is direct object, the what clause can agree with either a singular or plural verb: What I need is names and addresses and What I need are names and addresses are both Standard, although the notional attraction from the plural predicate nominatives will tend to make the plural are the choice. Nearly every other use of the what clause requires a singular verb, as in What we need to know today is how much time is left [how many hours are left]. See NOTIONAL AGREEMENT.
