Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
RESTRICTIVE AND NONRESTRICTIVE CLAUSES
These are restrictive and nonrestrictive modifiers in clause form: The convertible that had its top down was mine (restrictive); The convertible, which had its top down, was mine (nonrestrictive). The restrictive clause in the first sentence specifies which convertible is under discussion; the nonrestrictive clause in the second sentence merely adds interesting but not crucial detail about the only convertible under discussion. Using or omitting the commas makes a difference, reflecting as it does the differing intonation patterns the spoken language provides for restrictive and nonrestrictive modifiers. But see also THAT (4) for a discussion of the distribution of these two relative pronouns. See also RELATIVE CLAUSES.
