In recent years there has been a trend toward the omission of periods in abbreviations. This is particularly true of scientific and technical writing, but the practice has been spreading in general writing as well.
(a) Do not use periods with the following:
(b) Use periods
If a sentence ends in an abbreviation taking a period, only one period is used.
For further information on spacing, see 7.02 Spacing.
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:
Use the following abbreviations for non-military titles preceding or following personal names:
Use Ms. when referring to a woman unless a preference for Mrs. has been indicated. Although not an abbreviation, Ms. is written with a period, by analogy with Mr. and Mrs. Note that Miss is not an abbreviation and does not take a period.
Do not use Mr., Mrs., Ms., Dr. or Esq. with any other abbreviated title or with an abbreviation denoting an academic degree or honour:
Do not use the abbreviation Dr. and Rev. Dr. before the names of individuals who hold only honorary doctorates.
Saint is written out for names of persons revered as holy, but may be abbreviated in informal contexts and in lists and tables:
St. and SS. (plural) are the abbreviations used.
Abbreviate professional and official titles only when they are used with surnames preceded by first names or initials:
Note that there are spaces between each period and the following initial or name.
Even when used to address someone in correspondence, Rt. Hon., Hon. and Rev. must be preceded by the:
The honorary title "the Honourable" is used before the names of members of the Canadian Privy Council, lieutenant-governors and certain other officials. The title "the Right Honourable" applies for life to the governor general, prime minister and chief justice of Canada. See Department of Canadian Heritage, Precedence of Canadian Dignitaries and Officials.
Do not abbreviate Mayor, Vice-President, Professor and Father when used with personal names.
The salutation will vary depending upon the person addressed and the nature of the letter. The following are some appropriate salutations for various circumstances:
If the person’s name or title is not known, the expression To whom it may concern may also be used. It is not recommended that Mr., Mrs. or Ms. be used with a title as a salutation, as in "Mr. Premier."
The form preferred or used by the person being addressed or referred to should be retained if it is known. Otherwise, the following guidelines should be applied in order to ensure uniform and equal treatment of the sexes.
Dear Ms. Samuels:
Dear J. D. Simmonds:
Where the name of the addressee is not known, use the form "Dear Sir/Madam" or "Dear Madam or Sir."
Dear Members of the Rotary Club:
To the Consumer Relations Department:
To whom it may concern:
An alternative is to use the memo format and omit the salutation.
See also 10.17 Salutation or greeting.
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