1.  When should I use a professional title and when should I use a social title?

If the recipient has a professional title (such as Dr., Professor, or President), use that and the last name, followed by a colon.  (Use a comma only in less formal writing.)   Otherwise, address a male as Mr. and a female as Ms.

2.  How should I address a married couple when one person has a professional title such as Dr. and the other does not?

If the husband has the professional title, use Dear Dr. and Mrs. Smith. 

For the address line, use
     Dr. and Mrs. John Smith
     (address)

If the wife has the professional title, use Dear Dr. and Mr. Smith, or Dear Dr. Mary Smith and Mr. John Smith (her title and name first). 

For the address line, use
     Dr. Mary Smith
     Mr. John Smith
     (address)

3.  How should I address a married couple when both persons have professional titles such as Dr.?

Use Dear Drs. Smith, or Dear Drs. John and Mary Smith. 

For the address line, use
     Dr. John Smith
     Dr. Mary Smith
     (address)

4.  When should I use Mrs. or Miss rather than Ms.?

If you know that the recipient prefers a title other than Ms., use that title, as in Miss Edwards or Mrs. John Victor.  Otherwise, use Ms. 

5.  Should I use a period after Ms.?

Yes.  It is sometimes argued that the period should be omitted because Ms. is not an abbreviation but a made-up social title or courtesy title.  For just that reason, however, the preferred spelling is with a period.  Spelled with periods, both Mr. and Ms. have the appearance of parallel titles, and neither title denotes marital status.

The American Heritage Dictionary offers the following usage note regarding the origins of Ms.:  “Many of us think of Ms. or Ms as a fairly recent invention of the women’s movement, but in fact the term was first suggested as a convenience to writers of business letters by such publications as the Bulletin of the American Business Writing Association (1951) and The Simplified Letter, issued by the National Office Management Association (1952).” 

6.  When should I abbreviate titles?

Use abbreviations for Mr., Ms., Mrs., and Dr., but spell out professional, religious, and military titles such as Professor, Father, Sister, and Colonel.

7.  How should I address dignitaries?

Many dignitaries – such as governors, senators, representatives, and mayors – are addressed The Honorable [first name/last name] and, in the salutation, Sir/Madam: or Dear [title/last name]:.  But those conventions are complicated, so consult a style manual.

8.  What if I can’t determine the person’s gender by the name?

Fudge.  Use a professional title in place of Mr. or Ms., or use both names and no title, as in Dear Chris Parker.

9.  What if I don’t know the person’s name?

Whenever possible, take the time to find out the recipient’s name.  Letters addressed to an individual have greater impact.  But if that is impossible or impractical, you have four alternatives:  Use the formal Dear Sir or Madam (which is correct but may sound old-fashioned), use a title (as in Dear Account Supervisor or Dear Auditor), use the all-inclusive but impersonal To whom it may concern, or simply omit the salutation.

10.  Is Dear Sir correct?

Dear Sir is acceptable when addressing a male reader or a group that you know to be all-male, though it sounds formal and perhaps old-fashioned, as noted above.  It should not be used to address a group that may include both males and females.  Instead, use Dear Sir or Madam or one of the other three alternatives suggested in point 9 above.

11.  When is it appropriate to omit the salutation?