Attached Link: http://newark.rutgers.edu/~jlynch/Writing/a.html

*Apostrophe. The most common way to form a possessive in English is with apostrophe and s: "a hard day's night." After a plural noun ending in s, put just an apostrophe: "two hours' work" (i.e., "the work of two hours"). If a plural doesn't end in s children, men, people plain old apostrophe-s: "children's," "men's," "people's." It's never "mens'" or "childrens'." There's also the opposite case: when a singular noun ends in s. That's a little trickier. Most style guides prefer s's: James's house. Plain old s-apostrophe (as in James' house) is common in journalism, but most other publishers prefer James's. It's a matter of house style. Note that the possessives of pronouns never get apostrophes: theirs, not their's; hers, not her's; its, not it's. See It's versus Its. Apostrophes are sometimes used to make acronyms or other abbreviations plural (another matter of a local house style). My preference: don't use apostrophes to make abbreviations plural not "They took their SAT's," but "They took their SATs." The only exception is when having no apostrophe might be confusing: "Two As" is ambiguous; make it "Two A's." Never use apostrophes as quotation marks to set off words or phrases (unless you need a quotation within a quotation). To refer to a decade, don't use an apostrophe before the s. Refer to the 1960s or the '60s (the apostrophe indicates that "19" has been omitted), not the 1960's or (worse) the '60's.*