Acronyms & Abbreviations: First-Use Rules and Style
Grammar6 min readMay 2026
Acronyms speed up writing — until they slow down readers. The trick is knowing which ones to introduce, which to assume, and how to handle plurals, possessives, and punctuation. This guide covers the rules that hold across AP, Chicago, and MLA style.
Common Style Differences
Item
AP Style
Chicago
MLA
U.S. / US
U.S. (with periods)
US (no periods)
US (no periods)
Plural acronyms
APIs (no apostrophe)
APIs (no apostrophe)
APIs (no apostrophe)
First-use spellout
Required unless universal
Required for unfamiliar
Required unless universal
"e.g." and "i.e."
Avoid in body
OK with commas
OK with commas
Doctor / Mr.
Dr., Mr. (periods)
Dr., Mr. (periods)
Dr. Mr (no period UK)
The First-Use Rule
Spell out the full term, follow with the acronym in parentheses, then use the acronym for the rest of the document: "the World Health Organization (WHO) recommends…". Reset the rule for new sections in long documents — readers don't always start at the top.
Plurals and Possessives
Plural: add lowercase 's' — URLs, CEOs, APIs.
Possessive: add apostrophe-s — the CEO's announcement, the API's response.
Plural possessive: apostrophe after the s — the CEOs' meeting.
Never: CEO's as a plural — this is the most common error in business writing.
When to Skip Acronyms
The term appears only once or twice — just write it out.
The audience won't recognize the spelled-out version either.
The acronym is offensive, controversial, or ambiguous in another field.
You're writing for a general audience and the acronym is industry jargon.
Check Your Copy's Clarity
Acronym-heavy text often scores poorly on readability — check yours.
Spell first, abbreviate later — the universal rule
On first mention, write the full term followed by the acronym in parentheses: "Application Programming Interface (API)". After that, use the acronym freely. This is the convention in journalism, scientific publishing, and every major style guide (AP, Chicago, APA, MLA). The exception is a tiny set of acronyms — NASA, FBI, HTML, URL — that are more recognisable than their expansions for the relevant audience.
Acronym vs initialism vs abbreviation
Acronym: pronounced as a word. NASA, RADAR, SCUBA, laser.
Initialism: pronounced letter-by-letter. FBI, HTML, USA.
Abbreviation: any shortened form, including contractions ("Dr.", "etc.", "vs.").
The distinction matters because acronyms take the article that matches the pronounced word ("a NASA mission") while initialisms take the article that matches the letter sound ("an FBI agent"). Getting the article wrong is a tell that the writer is not paying attention.
When to skip the expansion entirely
The acronym is the official name. There is no "Light Amplification by Stimulated Emission of Radiation" anymore — it is a laser.
The audience is technical and the term is core to the field. "TCP/IP" in a networking document.
Repeated use in a small document. After the third "API" on a page, expansion becomes noise.
Plurals — add a lowercase "s" without an apostrophe: "APIs" not "API's". Apostrophes are for possessives ("the API's response").
Periods in acronyms — modern style drops them. "USA" not "U.S.A.", "FBI" not "F.B.I.". Exception: lowercase initialisms ("a.m.", "e.g.").
Made-up internal acronyms in customer-facing copy. Your team knows what an "RPO" or "SDM" is; the customer does not.
Style consistency > style correctness. Pick a house style and apply it everywhere. A document that uses "USA" once and "U.S.A." twice looks unprofessional even if both are technically allowed.
Frequently Asked Questions
Acronyms are pronounced as words (NASA); initialisms letter by letter (FBI).
Yes for most acronyms — exceptions for universally known ones like FBI or HTML.
Add lowercase 's' without apostrophe — APIs, CEOs, URLs.
Modern usage drops them in most acronyms; some style guides keep U.S. and Dr.
Spell out on first use even for technical audiences and keep a glossary.