The Chicago Manual of Style distinguishes between acronyms, initialisms, and contractions, all under the umbrella of abbreviation. They are defined as follows: acronym refers only to terms based on the initial letters of their various elements and readable as single words (NATO, AIDS), initialism to terms read as a series of letters (BBC, ATM), and The Chicago Manual of Style, 15th Edition (2003) comes out strongly in favor of spelling out United States (rather than abbreviating it) when the term is used as a noun and not an adjective: 15.34 "U.S." or "US." Except in scientific style, U.S. traditionally appears with periods. abbreviations take no apostrophes: the GNPs of the United States and Japan. Possessive abbreviations do take apostrophes: DoD's report, the USPS's budget. With Definite Article. Whether to use "the" before an abbreviation is not established by any reliable rule. In general, an abbreviation that starts with a vowel (or with a consonant Additionally, the Chicago Manual of Style recommends its use for all citation of legal material. What follows is a summary of the basics. and the United States District Court for the Western District of Pennsylvania is abbreviated to "W.D. Pa." Sources and courts tend to have official abbreviations for this purpose, USA is an abbreviation for the United States of America. US tends to be the more popular way to abbreviate United States. The Chicago manual of style. Publication: Chicago : The University of Chicago Press, 2017. United States. Authorship -- Style manuals. Authorship. Authorship A. Garner Punctuation Spelling, distinctive treatment of words, and compounds Names, terms, and titles of works Numbers Abbreviations Lanugages other than English Mathematics in Political Science Reference Guide for Chicago Manual of Style . STYLE . Abbreviations . See 15.45. Use abbreviations at first occurrence of a term [e.g., fiscal year (FY)]. May use both abbreviation and full term throughout text. Prioritize "that is," for "i.e.," and "for example," for "e.g.," (always follow with a comma) (5.202 To make matters worse, some well-respected style guides recommend the first style, and others prefer the second. Finally, the choice partly depends on whether U.S. is functioning as a noun or as an adjective. Here is my advice: 1. In formal writing, spell out the two words as "United States," as in the example below. This is always appropriate. The only difference between footnote and endnote citation style is that the first and last names of the author are switched, so that the bibliography is organized alphabetically by last name. For example: In-text footnote: Fred L. Block. The Origins of International Economic Disorder: A Study of United States This applies to Chicago Manual of Style and the American psychological Association Style (APA). The two-letter, no-period state abbreviations recommended by the U.S. Postal Service (see "Postal Abbreviations" in the chart below) should always be used where a Zone Improvement Plan (ZIP) code follows.