When to use abbreviations
In general, abbreviations are most appropriate in tabular matter, notes, bibliographies, and parenthetical references.To the greatest extent possible, abbreviations should be kept out of running text, except in technical matter.
Even in regular prose, a number of expressions are almost always abbreviated and may be used without first spelling them out. (Many of these will be listed as main entries of the latest edition of Webster's.) For example: DNA, GPS, HMO, HTML, IQ, JPEG, laser, Ms., NASA.
Others, though in more or less common use (CGI, FDA, HVAC, MLA) should generally be spelled out at the first occurrence — at least in formal text — as a courtesy to those readers who might not easily recognize them.
The use of less familiar abbreviations should be limited to those terms that occur frequently enough to warrant abbreviation — roughly five times or more within the article or chapter — and the terms must be spelled out on their first occurrence.
“a” or “an” before an acronym
When choosing between “a” and “an” before an acronym that is articulated letter by letter (that is, an anagram such as “UPS” as opposed to “NASA,” which is articulated as a word), consider how the first letter sounds when said out loud. For example: a USDA spokesperson but an EPA spokesperson.
General abbreviations (etc., e.g., i.e.)
General abbreviations such as etc., e.g., and i.e. are preferably confined to parenthetical references and used only when necessary.
i.e. and e.g. The first is the abbreviation for id est (“that is”); the second is the abbreviation for exempli gratia (“for example”). English equivalents are preferable in formal prose; Chicago style is to use these two-character abbreviations only within parentheses or in notes. Always put a comma after either of them. (Chicago 16, 5.220, p.284)
Always spell out the names of states when they stand alone in text, and preferably (except for DC) when following the name of a city: She works at a coffee shop in Somerville, Massachusetts.
Use the two-letter postal codes (listed below) in bibliographies, tables, lists, or when providing a party affiliation.