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|
For the confused and curious, here are some "Apostrophes for Dummies" websites: