The general rule is to omit periods in an abbreviation with two or more capital letters. Abbreviations such as UK and UN are therefore generally written without periods, except in certain styles, as discussed below. The abbreviation US (or U.S.) may be written with or without periods, depending on the style you follow, while EU is always written without internal periods.
Periods in UK and UN
Don’t use periods in the abbreviations UK and UN (for United Kingdom and United Nations) unless the style you follow specifically recommends it. Style manuals differ in their recommendations:
- The AP Stylebook mandates the use of periods—U.K. and U.N., no spaces—except in headlines, where it recommends using these abbreviations without periods, presumably to conserve space.
- The APA Publication Manual recommends using periods in U.K. (no space) but not in UN.
- Other style manuals, such as the Chicago Manual of Style, MLA Handbook, and New Oxford Style Manual recommend omitting internal periods in these abbreviations: UK and UN, without periods.
- Style guides of British news publishers, including the BBC News style guide and Guardian and Observer style guide, recommend omitting periods: UK and UN.
- Dictionaries such as Merriam-Webster, Oxford, and Cambridge also omit the periods in the spelling of these abbreviations (UK, UN), with Oxford listing U.K. and U.N. as variant US English spellings.
Whether to use periods in abbreviations is a matter of style rather than grammar. Style manuals differ in what they recommend because they each respect the conventions of their field (journalism, language, humanities, cultural studies, social sciences, hard sciences, etc.).
- Correct: Chicago, MLA style: Only two UK delegates attended the UN conference on climate change.
Correct: APA style: Only two U.K. delegates attended the UN conference on climate change.
Correct: AP style: Only two U.K. delegates attended the U.N. conference on climate change.
If you use periods in an abbreviation, don’t use a space after the period: U.K. and U.N., not U. K. and U. N.
Periods in EU
Don’t use periods in the abbreviation EU for European Union. Most, if not all, style authorities recommend writing EU without internal periods, including the Chicago Manual of Style, MLA Handbook, AP Stylebook, APA Publication Manual, various standard dictionaries, and the Interinstitutional Style Guide of the European Union.
- Incorrect: The E.U. has 24 official languages.
Correct: The EU has 24 official languages.
Periods in US and USA
The abbreviation for United States has traditionally been written with periods (U.S.) but is now most often written without periods (US) in formal styles.
- The Chicago Manual of Style and MLA Handbook now recommend omitting the periods: US.
- The APA Publication Manual and AP Stylebook still mandate the use of periods (U.S.) except in headlines, as does the U.S. Government Publishing Office Style Manual.
- Periods are omitted in British style, as suggested by the New Oxford Style Manual, BBC News style guide, and Guardian and Observer style guide.
- Standard dictionaries such as Oxford and Merriam-Webster list the abbreviation as US (without periods), with U.S. (with periods) as a variant spelling.
Both styles are acceptable: U.S. and US. In formal writing, the periods are now omitted unless specifically required by the style guide you follow.
The abbreviation US may be written with or without periods, depending on the style you follow, but don’t use periods in USA.
Periods in abbreviated names of organizations
Don’t use periods in organization names that are abbreviations or acronyms (which are abbreviations pronounced as words) unless that organization’s style demands it. These abbreviated names usually contain capital letters and do not require periods.