Definition (from Random House Unabridged CD-ROM Dictionary): the sign (') , as used: to indicate the omission of one or more letters in a word, whether unpronounced, as in o'er for over, or pronounced, as in gov't for government; to indicate the possessive case, as in man's; or to indicate plurals of abbreviations and symbols, as in several M.D.'s, 3's.

Note that when a word ends in "s", the apostrophe (indicating possesion) is after the "s". Example: the house belonging to John Smith is John Smith's house; but the house belonging to all of the Smiths is the Smiths' house.

Also, in American English, one uses an apostrophe to indicate plural forms of abbrevations (shortened words), but not plural forms of acronyms (when the first letters of a long name have been grouped to form a "word"). Example: There are two ETHs in Switzerland which confer Ph.D.'s. I have found no counsel as to what to do if one has a possesive plural abbreviation. My own choice would be to say: "The Ph.D.'s careers..." (that is, the many careers of the many people who have Ph.D.'s.) In contrast, "The Ph.D.'s' careers...." or "The Ph.D.s' careers...." look odd.

In British English, abbreviations are not punctuated. So the American "Dr." becomes "Dr" in British. Therefore the plural would be "Dr.'s" in American and "Drs" in British. The possesive plurals would be "Dr.'s" in American and "Drs'" in British. Certainly British is clearer in this case.

1. What is the difference between "Lausanne Sharks win" and "Lausanne Sharks' win"?

The first one uses "win" as a verb, the second as a noun.

2. What are the possesive forms of "the hysteresis of the piezo" and "the nonlinearities of the images"?

the piezo's hysteresis, the images' nonlinearities

3. If I return to school for five more years I could earn another Ph.D. Then I would have two Ph.D.'s. {American} (or, two PhDs. {British})

4. Referring to the labs of two people: There are many instruments in the two Ph.D.'s {American} laboratories. (or, the two PhDs' laboratories {British})

5. Our lab used to have only one SPM. Now we have two SPMs. The two SPMs' capabilities are very good. But today, the older SPM's laser doesn't work.

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Created April 30, 1998, by Nancy Burnham and Fred Hutson.