When writing down the time, the use of a.m. and p.m. should include a period after each letter. Use a.m. for morning hours and p.m. for afternoon and evening hours. When writing the days or months, abbreviations are only acceptable when space is limited. Then, the first three letters should be used, followed by a period. When using era designations like B.C. or A.D. the letters should be written in capital letters. Use A.D. before the year number and B.C. after the year number.
When writing addresses, abbreviations should not be used at all when the address is being written in the body of the copy. However, when written on an envelope or mailing list, using appropriate abbreviations for the type of roadway, the compass direction, and the state are appropriate. State abbreviations must be the U.S. Postal Service approved format of two letters and no periods. When abbreviating a country, the common letters for the abbreviation should be used with periods. For example, United States is properly abbreviated U.S. rather than US.
When writing a person's proper title, abbreviations are acceptable for common titles like Dr., Mr. and Mrs., but should not be used for professional titles like Senator, Representative, or Professor, except where space does not allow the full word. For post titles, Jr. and Sr. are common and use periods but a comma should not be used to separate them from the name. Most degrees, like M.D., D.D.S., and Ph.D. are also written with periods. However, some editor's manuals recommend not using the periods. When abbreviating famous people, use all three initials without periods like LBJ, JFK, or MLK.
The abbreviation R.S.V.P. should be used with a phone number and a date to require a response. Latin abbreviations like e.g., which means "for example," and i.e., which means "in other words," should be in lower cases and followed with a comma, unless ending the sentence. Abbreviations for common objects, corporations, and familiar institutions are capitalized without the periods. Acronyms are different from abbreviations but are used in the same manner. They also are capitalized with no periods.