When writing about the United States, should you write US or United States or U.S. or what? Here’s the rule according to the Chicago Manual of Style 16th edition, 10.33:
In running text, spell out United States as a noun; reserve US for the adjective form only (in which position the abbreviation is generally preferred).
If that sounded confusing, don’t worry. Let’s break it down. In running text, spell out United States as a noun.
Example: I would rather spend my vacation going to Europe than staying here in the United States.
Example: I like living in the United States.
In these examples you’re using United States as a noun (ie. person, place, or thing). Now: Reserve US for the adjective form only.
Example: US foreign policy
Example: US beaches
Pretty simple once you know the rule, right? It’s probably not the most important rule, but it will come in handy if you’re writing or editing a book about the United States!