The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
When an abbreviation ending in a period is followed by a punctuation mark like a comma, colon, semicolon, or dash, use both the period and the punctuation mark. But use only a single period after an abbreviation at the end of a sentence.
If an abbreviation ending in a period also ends a sentence, use just one period, not two. With parentheses, place a period after the closing parenthesis to end the larger sentence.
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.
Use “which” to introduce a description. As a relative pronoun, “which” connects a relative clause to the noun it describes. Differences exist between American and British usage.
Use “that” as a relative pronoun in restrictive or defining clauses, which present information essential to meaning. Don’t use a comma before “that.”