Registered Trademarks

Q. “ZIP Code” is trademarked and essentially an invention by the USPS. They created it with a capitalized “C.” Why do you insist that this “C” is rendered in lowercase?

A. You are right that the United States Postal Service writes “ZIP Code” with a capital C—and ZIP in all caps. You’re also right about us. Though CMOS doesn’t specifically rule on how to capitalize the term, it has rendered it as “zip code” (all lowercase) since it first appeared in the Manual in 1982 (in the 13th ed.). Our editors at that time would have been conforming such passing mentions to the main entries in Webster’s New Collegiate Dictionary (8th ed., Merriam-Webster, 1973). The eighth Collegiate had “zip code n, often cap Z&I&P” (those are ampersands between the letters Z and I and P)—the same as in the current dictionary from Merriam-Webster. (Compare the entry for “Kleenex,” which is listed with a capital K and noted as a trademark.)

Merriam-Webster and CMOS aren’t the only influential resources that don’t fully conform to Postal Service usage. The 2020–2022 edition of the Associated Press Stylebook says to write “all-caps ZIP for Zone Improvement Plan, but always lowercase the word code.” But don’t despair. We are filing your question with the many other suggestions from our readers for future editions of CMOS. Though “zip code” may remain our preferred style for general references to the numeric locator that in the US has become practically synonymous with “neighborhood,” we’ll make a note to acknowledge the trademarked styling as well.