U

U.K.

All right to use as an abbreviation for United Kingdom as an adjective or as a noun, but only when space is limited. Otherwise, do not use unless you have no other choice.

U.S.

All right to use as an abbreviation for United States but only as an adjective and only when space is limited. Otherwise, do not use unless you have no other choice. Do not use US, USA, or U.S.A.

If you use the abbreviation, you do not have to spell out U.S. on first mention. Always use periods and no space.

Spell out United States as a noun except when third-party, legally required content specifies otherwise.

un-

In general, do not hyphenate words beginning with un-, such as undo and unread, unless it is necessary to avoid confusion, as in un-ionized, or unless un- is ...

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