As you already know, apostrophes are used to form both contractions—two words collapsed into one—and possessives. Handily, we can virtually ignore the issue of contractions here, since they are so easily understood and are rarely used in technical writing. With possessives, the apostrophe is used, typically in combination with an “s,” to represent that a word literally or conceptually “possesses” what follows it.

a student’s paper              the county’s borders
a nation’s decision            one hour’s passing

Apostrophes with Words ending in “s”

Although practices vary, for words that already end in “s,” whether they are singular or plural, we typically indicate possession simply by adding the apostrophe without an additional “s.”

Illinois’ law                    Student Affairs’ office
Mars’ atmosphere        interviewees’ answers

Apostrophes with Acronyms and Numbers

In technical writing, acronyms and numbers are frequently pluralized with the addition of an “s,” but there is typically no need to put an apostrophe in front of the “s.” Therefore, “SSTs” (sea surface temperatures) is more acceptable than “SST’s” when your intention is simply to pluralize. Ideally, use the apostrophe before the “s” with an acronym or a number only to show possession (i.e., “an 1860’s law”; “DEP’s testing”) or when confusion would otherwise result (“mind your p’s and q’s”).

Possessives without the Apostrophe

Convention, frequency of usage, and—to be honest—the economy of advertising, sometimes dictate that the apostrophe is dropped. In proper names that end in “s,” especially of geographic locations and organizations, the apostrophe is often omitted. And in everyday combinations where possession is automatically understood, the apostrophe is often dropped.

United States government               Hells Canyon
Veterans Highway                            Harpers Ferry
mens room                                       Johns Hopkins University