The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Initialisms and acronyms generally contain all capital letters, although Latin, technical, and scientific abbreviations are often lowercased. Contracted social titles and proper nouns are capitalized.
The article “the” is generally omitted with acronyms of proper nouns (NATO, NASA) but used with initialisms (the US, the UK).
Use “a” or “an” before an abbreviation depending on how it is pronounced not written. If it starts with a consonant sound, use “a”; if it starts with a vowel sound, use “an.”
Avoid starting a sentence with an abbreviation. Use the full form instead, or rephrase. An acronym (abbreviation pronounced as a word), however, is acceptable in this position.
Provide the full form of an abbreviation at first use. Write the complete term and enclose the abbreviation in parentheses, unless the term is better known in its abbreviated than full form.
An abbreviation is the shortened form of a term. It usually comprises the first letters of the words of a phrase or is the contracted form of a word.
BCE and CE are religiously neutral alternatives to BC and AD. While BC, BCE, and CE appear after the date (223 BC), AD appears before (AD 1776). Periods are optional and generally omitted.
Don’t enclose the abbreviation for a time zone in commas, and don’t use periods after the letters of the abbreviation. Full forms may or may not be capitalized. With an international audience, UTC offsets are often preferred.
Periods are used when the abbreviations for time are lowercased (a.m., p.m.) but omitted when they are capitalized (AM, PM).
The abbreviations “a.m.” and “p.m.” are generally lowercased in running text, though they may also be capitalized. When lowercased, the letters are followed by periods; when capitalized, periods are omitted.