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

*I thought everyone knew that the apostrophe refers to ownership. Why do I still read things like "The car was from the 50's" and "Houses are selling in the low 200's"? Shouldn't it be "The car was from the 50s" and "Houses are selling in the low 200s"*

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.