You’ve surely heard of preventive lawyering, preventive maintenance, and preventive medicine. What they denote is related to the old adage: a stitch in time saves nine.
But an extra stitch is often added to the word: the additional syllable in *preventative. That’s what you hear if you listen to many TV commentators. One popular announcer (who mustn’t be named) actually says *preventative even when the caption beneath her reads preventive.
Both forms of the word date back to the 1600s. Preventive is (1) older, (2) more common, and (3) more correctly formed by analogy to the noun (prevention, of course, not *preventation). That’s why usage commentators have long recommended it over the longer form. Few today go as far as Richard Grant White, who in 1870 wrote that *preventative shows “an utter want of education and a low grade of intelligence.” Richard Grant White, Words and Their Uses 229 (1870).
Why do so many people add the extra syllable? Perhaps they analogize the word to representative (whose noun is representation, not *represention), qualitative, quantitative, or tentative.
What do we say at LawProse? If you want to be authoritative, say and write preventive.
The Chicago Manual of Style 348 (17th ed. 2017).
Roy H. Copperud, American Usage and Style: The Consensus 304 (1980).
Garner’s Modern English Usage 727–28 (4th ed. 2016).
THE FUNNY ASTERISK: Did you notice the asterisk prefacing certain words above? That’s a convention that linguists and other writers on language use to signal nonstandard forms—ungrammatical constructions and words not considered part of Standard English. That’s all. It’s convenient, don’t you think? Cheers.