Mrs., Mr., Ms., Prof., Dr., Gen., Rep., Sen., St. (for Saint)
Notice that Miss is not an abbreviation, so we don't put a period after it. Ms. is not an abbreviation, either, but we do use a period after it — probably to keep it consistent with Mr. and Mrs.
The plural of Mr. is Messrs. (We invited Messrs. Carter, Lincoln, and Ford.) The plural of Dr. is Drs. (We consulted Drs. Carter, Lincoln, and Ford.) The plural of Mrs. is Mmes or Mmes. (with or without the period).
In most formal prose, we do not use titles, abbreviated or otherwise, with individuals. Ms. Emily Dickinson is simply Emily Dickinson, and after the first use of her full name, Dickinson will do (unless we need Emily to avoid confusion with other Dickinsons).
The abbreviations Rev. and Hon. (for Reverend and Honorable) are not, strictly speaking, titles; they are adjectives. In informal language or when we're trying to save space or make a list, we can write Rev. Alan B. Darling and Hon. Francisco Gonzales. In formal text, we would write "the Reverend Alan B. Darling" and "the Honorable Francisco Gonzales" (i.e., it's not a good idea to abbreviate either Reverend or Honorable when these words are preceded by "the"). Incidentally, we cannot say "We invited the reverend to dinner" and only a cad would invite "the rev."
Sr., Jr., Ph.D., M.D., B.A., M.A., D.D.S.
These are standard abbreviations, with periods. The APA Publication Manual recommends not using periods with degrees; other reference manuals do recommend using periods, so use your own judgment on this issue. All sources advise against using titles before and after a name at the same time (i.e., she can be Dr. Juanita Espinoza or Juanita Espinoza, PhD, but she cannot be Dr. Juanita Espinoza, PhD). And we do not abbreviate a title that isn't attached to a name: "We went to see the doctor (not dr.) yesterday."
The Chicago Manual of Style recommends not using a comma to separate the Jr./Sr./III from the last name, but you should follow the preferences of the indivdual if you know those preferences. If you list a "junior" with his spouse, the "Jr." can go after both names, as in "Mr. and Mrs. Arthur C. Banks Jr." or "Mr. Arthur C. Banks Jr. and Gloria Banks — but not Arthur C. and Gloria Banks Jr. You should avoid using a "Jr." or "Sr." when you have only the last name — Mr. Banks Jr.
Notice that U.S.A. can also be written USA, but U.S. is better with the periods. Also, we can use U.S. as a modifier (the U.S. policy on immigration) but not as a noun (He left the U.S. U.S.A.).
Terms of mathematical units: 15 in., 15 ft, 15 kg, 15 m, 15 lb
Generally, you would use these abbreviations only in technical writing. There is a space between the number and the abbreviation. Notice that we do not put an s after such abbreviations even when the plural is indicated. Also, we do not use a period with such abbreviations except for in. when it might be confused with the preposition in.
When the term of measurement is used as a modifier, we put a hyphen between the number and the term of measurement: a 15-ft board, a 6-lb line, etc.
Long, common phrases, such as IQ (Intelligence Quotient), rpm (revolutions per minute), mph (miles per hour), and mpg (miles per gallon).
Such abbreviations are acceptable even in formal academic text and may be used without periods.
Words used with numbers: He left at 2:00 a.m. She was born in 1520 B.C.
Either lower or upper case letters can be used with A.M., a.m., P.M., p.m. The abbreviation B.C. (before Christ) is used after the date; A.D. (anno domini, "in the year of the Lord") appears before the date. The abbreviations B.C. and A.D. are sometimes replaced with B.C.E. (before the common era) and C.E. (common era), both used after the date (although one must add that those abbreviations are neither widely used nor commonly understood). Sometimes you will see 790 BC and AD 78 written without periods and written in SMALL CAPS. Note that many style books are now recommending SMALL CAPS for all appearances of acronyms, such as NAACP or NCAA. The effect of this practice is to allow the acronym to blend more smoothly with the rest of the text.
It is considered bad form to use these abbreviations without a specific number attached to them: "We'll do this in the a.m." or "We'll do this tomorrow a.m."
Common Latin terms: etc. (et cetera — and so forth), i.e. (id est — that is), e.g. (exempli gratia — for example), et al. (et alii — and others).
Except in the business of formally citing material you've used in research, it's a good idea not to use et al. when you mean "and others." And don't use etc. as a lazy person's way of getting out of work. Spell out the word versus unless you're reporting game scores, when you would use vs.; when you're citing legal documents, use the abbreviation v.
Names of states and territories in references and addresses, but not in normal text. Abbreviations accepted by the
Abbreviate "Saint" in U.S. place names, as in St. Louis and St. Petersburg, Florida, and the St. Lawrence River. For the same word in other countries, you might have to consult a good dictionary (one that contains place names): St./Saint Martin's in the Fields, Saint Moritz, Saint Lucia, Mont-Saint-Michel, Saint Petersburg (Russia). When the word Saint is used to refer to a holy person, spell out the word — Saint Theresa, Saint Francis of Assisi. If an institution is named after a saint, spell out the word Saint unless you have some reason to save space — Saint Francis Hospital, Saint Joseph College, Saint Joseph's University. It is wise, as always, to consult the actual institution. Colleges, universities, and hospitals named after Saint Mary are about evenly divided between St. and Saint, but in formal situations, Saint seems to be favored more frequently.
(In formal academic prose it is considered bad form to abbreviate words simply to save space, time, or energy.)
Abbreviations of units of measure are written without periods (with the exception of "in" when it could be confused with the preposition). We use periods for most lower-case abbreviations such as e.g. and i.e. and c.o.d. For very common abbreviations, leave out the periods, as in rpm and mph. When an abbreviation with a period ends a sentence, that period will suffice to end the sentence: He lives in Washington, D.C. Suffixes for people's names require periods: Joe Smith Jr. lives in Erie. In formal text it is not a good idea to abbreviate military titles — Lieutenant Colonel Chester Piascyk — but in informal text Lt. Col. Chester Piascyk would be acceptable. (Note the space after "Lt.") Academic degrees can be written with periods or not, but don't insert spaces — Ph.D. or PhD, M.B.A. or MBA — within the degree.
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There is a difference between acronyms and abbreviations. An acronym is usually formed by taking the first initials of a phrase or compounded-word and using those initials to form a word that stands for something. Thus NATO, which we pronounce NATOH, is an acronym for North Atlantic Treaty Organization, and LASER (which we pronounce "lazer"), is an acronym for Light Amplification by Stimulated Emission of Radiation. FBI, then, is not really an acronym for the Federal Bureau of Investigation; it is an abbreviation. AIDS is an acronym; HIV is an abbreviation. URL is an abbreviation for Uniform Resource Locator (World Wide Web address), but many people pronounce it as "Earl," making it a true acronym, and others insist on pronouncing it as three separate letters, "U * R * L," thus making it an abbreviation. The jury is still out. (I vote for Uncle Earl.)
It appears that there are no hard and fast rules for using periods in either acronyms or abbreviations. More and more, newspapers and journals seem to drop the periods: NAACP, NCAA, etc. Consistency, obviously, is important.