The formation of plurals and possessives is usually pretty straightforward, except in the case of abbreviations and acronyms. The following rules should help.

To form the plural of an abbreviation, a number, or a capital letter used as a noun, simply add an 's' to the end.

• A group of MPs
• The late 1940s
• Mind your Ps and Qs

To form the plural of an abbreviation with periods, a lowercase letter used as a noun, and abbreviations or capital letters that would be ambiguous or confusing if the 's' alone were added, use an apostrophe and an 's'.

• A group of M.P.'s (see note below)
• The x's in the equation
• Sending SOS's

Note that, in Canada, periods are not generally used in abbreviations, with the exception of truncated words (such as Inc.) the abbreviation of certain Latin terms (such as e.g. and i.e.), and geographic names (such as U.S.A.—although, in some quarters, USA is accepted.) In the plural of an abbreviation with periods, you may choose to use an apostrophe for a more attractive appearance (as is the American style), or omit the apostrophe (as is the British style). Be careful that the result does not cause confusion (e.g., "the M.P.'s meeting" could refer to a meeting attended by MPs, or a meeting hosted by an MP.)

To form the possessive of most abbreviations, add an apostrophe and an 's'.

• The MP's constituency
• The YMCA's facilities

To form the possessive of a singular abbreviation ending in 's', add an apostrophe and an 's'. When an abbreviation ends in an 's' used to form the plural, add an apostrophe only. Note, however, that these situations are better avoided.

• CSIS's headquarters (better: CSIS headquarters)
• an increase in MPs' salaries (better: an increase in the salaries paid to MPs)

Metric symbols, units of measure, and certain other abbreviations do not change in the plural.

• 1 km
• 100 km
• ch. (chapter)
• ch. (chapters)

A small number of abbreviations have unique plural forms. For instance:

• l. (line); ll. (lines)
• p. (page); pp. (pages)