Attached Link: From the good folks at the Purdue OWL News

The apostrophe has three functions in English: it indicates the possessive case (such as John's letter), it stands in for missing letters in a word (such as "don't"), and it can also be used to mark the plural of numbers, acronyms, or letters, such as in the example given in your question. The rationale given for this third use is that it helps to make the plural form clear to a reader. For example, the plural form of the letter A in the sentence "Jane got all As in school" might be read as the word "As" rather than the plural of the letter A. If the letter was a lowercase letter, such as in the sentence "The typesetter got out a box of a's and b's and began setting up the press," you can see how the confusion would be even more apparent if apostrophes were not used to show the plurals of the lowercase letters. However, the use of the apostrophe to show the plural of these specific forms is rapidly declining in usage in American English. So, while it is correct to write "Houses are selling in the low 200's," it is also correct to write "Houses are selling in the low 200s." The second form is becoming the more commonly accepted form.

In the Oxford Dictionary of American Usage and Style, Bryan Garner discusses the use of the apostrophe for plural forms, and recommends the use of an apostrophe to show the plural of lowercase letters while encouraging the adoption of a simple -s ending to show the plural of capital letters, acronyms, and numbers. In The Prentice Hall Reference Guide to Grammar and Usage, Muriel Harris concurs with Garner's recommendation for forming the plural of lower case letters, and says "For capital letters, abbreviations without periods, numbers, symbols, and words used as words, the apostrophe before the -s is optional if the plural is clear." (p. 170) In any case, consistency in the use of apostrophes in pluralizing letters, numbers, and acronyms is most important. So choose the method which appears most correct to you and stick with it. As always, apostrophes are never used to form the plural of nouns that are not letters, numbers, or acronyms, or to indicate the tense of a verb.

For more about apostrophe use, see our handout at http://owl.english.purdue.edu/handouts/grammar/g_apost.html.

Can you please tell me how to form the possesive of a name ending in "s"? I was taught in school to put an apostrophe after the final s. Recently I read Stephen King's book "On Writing". He commands the reader to always use " 's ". I used my old doctor's name Cavins in a story. Which is correct: Cavins' or Cavins's to show possession?

I'm not claiming this is the ultimate authority but this makes sense to me:

"There seems to be some judgment required when forming the possessive of a noun that already ends in "s." Usually, add "'s" (another syllable): "bus's" or "boss's." However, quite often it would seem to depend on just how you personally say the possessive. My dictionary says that both Jones' and Jones's are correct. Another source says that boys' is correct, as it is normally pronounced as one syllable. In most of these cases, it would seem that you can get away with either apostrophe-s (usually preferred) or just the apostrophe (also acceptable)."

So I would think Cavins's, because it sounds like a separate syllable to me.

The wonderful thing about English is that it is very fluid, so you may find someone else will disagree with Mr. King and me, and I won't argue the point very hard.

I have to tell you the truth, I often rewrite the sentence to avoid the situation, so I will refer to "the house that belong to Calvins" rather than "Mr. Calvins's house."

But that's the coward's way out. ;)

The correct form historically: Cavins' However, language changes as people's use of it changes. Speak it out loud. Which would you say? If you would speak Ca-vins-es, then add the second "s" when you write it.

The ultimate answer on this capitalizing nouns ending in s is... There is no official right answer to this question. Be consistant, because that will make it easier for your copyeditor to change it to whatever that publishing house standard is, after you sell it. If you aren't planning to sell it, do whatever looks/sounds good to you.