an icon showing an apostropheWith possessives, the apostrophe is used 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 and with Plurals

Singular words whether or not they end in s, are made possessive by adding an apostrophe + s. For plural words, we typically indicate possession simply by adding the apostrophe without an additional s. However, a plural that does not end in an s (e.g., bacteria), we would add an apostrophe + s.

  • Illinois’s law
  • Mars’s atmosphere
  • interviewees’ answers
  • the bacteria’s life cycle
  • her professors’ office (an office shared by two of her professors; if it were just one professor we would write her professor’s office)
Note: Practices vary from style to style, so be sure to check the rules in your course’s discipline for this.


A contraction is a shortened phrase. He will becomes he’llare not becomes aren’t, would have becomes would’ve, and it is becomes it’s. In all of these cases, the apostrophe stands in for the missing letters.

You may find yourself being steered away from using contractions in your papers. While you should write to your teacher’s preference, keep in mind that leaving out contractions can often make your words sound over formal and stilted. (And don’t eliminate contractions in your papers just to up your word count!)

Note: Double contractions, like wouldn’t’ve or I’d’ve are considered non-standard and should be avoided in formal written language.

Some Common Errors

Now that we’ve learned about both contraction and possession, let’s take a look at some of the most common (or at least most called out) errors people make.

Its versus It’s

This rule also applies to your vs. you’re and their vs. they’re. The best way to use these correctly is to remember that possessive pronouns never have an apostrophe: if there’s an apostrophe with a pronoun, it’s a contraction, not a possessive.

Should’ve versus Should of

  • Should of, would of, could of
  • Should’ve, would’ve, could’ve

This mistake is due to the pronunciation. Out loud both of these phrases sound exactly the same. However, remember that the original phrase is should have, as in “I should have done that.” The phrase should of should never occur. Unfortunately, the only way to remember this is rote memorization (or perhaps a closer examination of the word of).

Acronyms and Numbers

In technical writing, acronyms and numbers are frequently pluralized with the addition of an apostrophe + s, but this is falling out of favor, and 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”).

When talking about a specific decade the 1920s should be shortened to the ’20s. Notice that the apostrophe curls away from the numbers, indicating that the missing characters originally appeared prior to the apostrophe.


Select the response from the list that best completes the sentence.

  1. “(Who’s/Whose) cookies are these?” May asked. At the same time, Russell ran into the room and yelled, “(Who’s/Whose) the person who took my cookies?”
  2. I don’t understand people who think that (its/it’s) ok to pour the milk in the bowl before adding the cereal.
  3. Before the (1860s/1860’s/1860s’), no one knew that heating a liquid would kill off bacteria.
  4. Everyone in town knew that (Trisha’s/Trishas’) stew was better than anyone (else’s/elses).
  5. All my (neighbor’s/neighbors’/neighbors) apple trees bloom before mine.