Hyphens, En Dashes, Em Dashes

Q. I have a hyphenation question that I wasn’t able to resolve after reading CMOS or the Q&A page on your website. I am in a debate with a fellow attorney about the proper hyphenation for the phrase “explicitly-defined” when used in the context of “an explicitly-defined rule governing adoptions.” My colleague insists there should be no hyphen between “explicitly” and “defined.” I think that there should be a hyphen between the two words.

A. The CMOS rule, which you can find at paragraph 7.82 of the sixteenth edition, is to leave such compounds open. An ly strongly signals adverb—and adverbs cannot modify nouns by themselves. No hyphen is needed, then, to warn that the next word is not a noun but rather an adjective. There’s no such thing as “an explicitly rule,” so there is no chance of misreading “explicitly defined rule.”

That said, it has long been the practice elsewhere—among British writers, for example—to hyphenate ly + participle/adjective compounds. And American writers a century ago—Edith Wharton comes to mind (“the pallour of her delicately-hollowed face,” describing an exhausted Lily Bart toward the end of The House of Mirth [1905])—seemed always to use such hyphens (or at least their publishers did). The reasoning behind this approach may be that the ly so strongly telegraphs another modifier that the two might as well tie the knot.

So it is a matter not of who is right or wrong but of whose rule you are going to follow. We think that added to our logic is the small victory of avoiding a hyphen.