abbreviations: periods and spacing
When shortened forms take periods, and how they are spaced.
In running text, use a period after a shortened title or ordinary clipping: Mr., Mrs., Ms., Dr., Jr., Sr., Inc., etc., Jan. Do not put a space inside those forms. Do not use periods in all-cap abbreviations of three or more letters (NASA, HTML, NPR, USA). For names of countries and international organizations written as two letters, put a period after each letter: U.S., U.K., U.N. Write US, UK, and UN without periods only in tables and tight labels. State postal codes use two letters with no periods (CA, NY). Latin short forms that are still read as clippings keep periods and use a comma after them when they introduce an example or restatement: e.g., i.e. Units of measure take no periods (km, lb, mph).
Examples
UseDr. Patel met Mr. Chen in Washington.
UseThe U.S. delegation arrived on Jan. 4.
AvoidDr Patel met Mr Chen in U. S. A.
UseBring a jacket, e.g., a windbreaker.
Some houses drop the periods (US, UK). Our recommendation is U.S. with periods in running text. Do not double the period at the end of a sentence that already ends in an abbreviation: She arrived at 9 a.m.