Q: I am writing a standard operating procedure for my company (hotels) that describes, among other things, how employees should deal with “walk ins”— guests who “walk in” without a reservation. Are terms like “check in,” “check out,” and “walk in” hyphenated?
A: When compounds like those are used as verbs, they’re generally two separate, unhyphenated words. But as adjectives and nouns, they’re either hyphenated or a single word.
Here’s our advice on how to write those terms, based on preferences given in the 10 standard dictionaries we regularly consult. (Some dictionaries may follow the preferred spellings with lesser-used variants.)
Verbs (no hyphens): “We’ll check in Friday and check out Monday, assuming they’ll let us walk in.”
Adjectives (hyphenated or one word): “The check-in clerk says checkout time is at noon, and they accept walk-in customers.”
Nouns (hyphenated or one word): “Our check-in was easy and so was the checkout, even though we were walk-ins.”
The verbs involved here are phrasal verbs, which are usually defined as a verb plus an adverb. The Chicago Manual of Style (17th ed.) gives “settle down,” “act up,” and “phase out” as examples. “A phrasal verb is not hyphenated,” the manual says, “even though its equivalent noun or adjective might be.”
The book illustrates this variability with the phrasal verbs “flare up” and “burn out.” Their equivalent adjectives and nouns are “flare-up” (hyphenated) and “burnout” (unhyphenated).
But as we wrote in 2009, the conventions of hyphenation change over time, and the tendency is for hyphens to disappear from familiar compounds. In 2019, we described the evolution of the verb “check out” as well as the noun and adjective “checkout.”
Many other compounds follow the “check out”/“checkout” pattern—the phrasal verb is two separate words but the adjective and noun are one. These include “break down,” “hold up,” “crack down,” “hand out,” “build up,” “back up,” “lay off,” “send off,” “send up” (to mock), and usually “close out.”
Many other compounds, for now at any rate, still follow the “check in”/“check-in” pattern—that is, the phrasal verb is two separate words but the adjective and noun are hyphenated. Some examples are “drop in,” “drive in,” “cave in,” “drop off,” “carry on,” “die off,” and usually “clean up.”
If you come across a compound that we haven’t mentioned, how can you tell whether the adjective and noun forms are hyphenated or one word? The easiest way is to check out the compound in an up-to-date dictionary.