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. The apostrophe is also used for general terms to indicate the singular possessive case.
|a student's paper||the county's borders|
|a nation's decision||one hour's passing|
|miner's inch||author's revisions|
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."
|Presidents' Day||Student Affairs' Office|
|Mars’ atmosphere||interviewees’ answers|
Apostrophes with Acronyms and Numerals
In technical writing, acronyms are frequently pluralized with the addition of an "s," but there is no need to put an apostrophe in front of the "s" in that your intention is simply to pluralize rather than show possession. When referring to decades, form the plural by adding an "s," but do not use the apostrophe in any position.
|the 1960s||the 1960's|
|she is in her 30s||she is in her 30's|
When numerals or letters serve as the name of something and an "s" is needed, use an apostrophe before the "s" to make it clear that the letters are not part of the name.
|Boeing 747's||Boeing 747s|
|mind your p's and q's||mind your ps and qs|
When Possessives are Implied without the Apostrophe
Convention and frequency of usage sometimes dictate that the apostrophe is dropped. In proper names that end in "s," especially of geographic locations, acamedic institutions, and government entities, the apostrophe is often omitted. Likewise, in everyday combinations and with acronyms where possession is automatically understood or contextually irrelevant, the apostrophe is not needed.
|United States government||Hells Canyon|
|Veterans Highway||Harpers Ferry|
|mens room||Johns Hopkins University|
|an FDA regulation||the NIOSH position|
|the Virginia legislation||an 1860 law|
For the confused and curious, here are some "Apostrophes for Dummies" websites: