Add only an apostrophe to the end of a plural noun that ends in "s" when denoting possession. For example, when showing possession with words like "sisters," "friends" or "foxes," the apostrophe should be the last element of the word: "the sisters' party," "the friends' luncheon" and "the foxes' den."
Add an apostrophe followed by an "s" to a plural noun that does not end with an "s." For example, in the case of "women," "children" and "people," add an apostrophe plus an "s": "the women's laughter," "the children's toys" and "the people's republic."
Add an apostrophe and an "s" to make lowercase letters plural. The most common example of this is shown in the expression "mind your p's and q's."
Add an apostrophe and an "s" to make acronyms that either contain lowercase letters and/or periods plural. For example, to make the acronyms "Ph.D.," "B.Sc." or "C.D." plural, add an apostrophe plus an "s": "Ph.D.'s," "B.Sc.'s" and "C.D.'s." Note that you can also write these acronyms without periods. In that case, simply add an "s" without the apostrophe to pluralize.