Add an s, but not an apostrophe, to form the plural of most abbreviations:
Use an apostrophe and s to form the plural of numerical names of aircraft ending in a single letter:
In cases where the resulting form would be ambiguous, add an apostrophe before the s:
Add an apostrophe and s to form the plural of abbreviations containing more than one period, and an s without an apostrophe, to form the plural of abbreviations with only one period. In the latter case, the s precedes the period:
The plurals of Mr. and Mrs. are irregular:
The plural forms of the abbreviations for certain bibliographic references are different:
Note that SI/metric symbols maintain the same form for both singular and plural and are written without periods, except at the end of a sentence:
An acronym is a pronounceable word formed from the first letters of a series of other words, such as NAFTA, NATO or GATT. An initialism is formed from the initial letters of a series of words and may not be pronounceable as a word. Examples are GST, RCMP, OECD and IDRC. The distinction is a fine one and is often overlooked in practice. Do not use periods or spacing between the letters of an acronym or initialism.
In general, acronyms are not preceded by the definite article:
Usage varies with respect to initialisms. Those representing the names of organizations generally take the definite article, while those representing a substance, method or condition do not:
The correct form of the indefinite article (a or an) to use before acronyms and initialisms is determined by the consonant or vowel sound of the initial syllable, letter or number. The following examples illustrate correct English usage. Note that ease of pronunciation is the key:
Use upper-case letters for acronyms or initialisms in their entirety, even if some of the component words or their parts are not normally capitalized—unless the organization concerned prefers lower case:
Acronyms (not initialisms) of company names formed by using more than the initial letters of the words they represent. Usually, only the first letter of the acronym is capitalized:
Initialisms are always fully capitalized:
Common-noun acronyms treated as fullfledged words, such as radar, laser, scuba and snafu, are written entirely in lower case without periods.
When using acronyms or initialisms such as SIN (social insurance number), PIN (personal identification number) or ISBN (International Standard Book Number) do not repeat the word number (e.g. "SIN number"). Either write the expression out in full or use the abbreviated form on its own.
© Public Works and Government Services Canada, 2013
TERMIUM Plus®, the Government of Canada's terminology and linguistic data bank
Writing Tools – The Canadian Style
A product of the Translation Bureau