Forming the possessive of nouns can be one of the more confusing rules in writing when it comes to words ending with s. Besides differences with plural and a few exceptions, the answer depends on who you ask.

To form possessive singular of noun, add ‘s

  • Augustus’s favorite book is Code Complete by McConnell.
  • The vampire’s fangs sank into the duchess’s neck.
  • Read Aaron Hillegass’s book on iOS programming.
  • The debris’s cloud smothered the ship.

This is the first rule appearing in The Elements of Style by Strunk and White, and according to the text there are exceptions for ancient proper names ending in -es and -is such as Moses’ laws, and “such forms as for conscience’ sake, for righteousness’ sake.“

One could avoid uncertainty by removing the need for an apostrophe as in laws of Moses.

To form possessive plural of noun, if noun ends in s then add apostrophe only, else add ‘s

  • The students’ questions were about GPU performance.
  • The women’s room is closed.
  • The Obamas’ reside in the White House.

I’ve seen writers leaving off the extra s such as, Dickens’ novel or Kansas’ laws. Are they right?

Notice that Strunk and White’s rule fits speech. It’s (nearly) common to pronounce an extra s when saying, Dickens’s novel, but not when saying, for righteousness’ sake, which becomes a tongue-twister when adding an extra s. However, pronunciation of Kansas’s laws may cause some stumbling, but not as bad as Kansas’s schools. Style guides try to take pronunciation into account resulting in exceptions and different approaches. For instance, Pocket Fowler’s Modern English Usage suggests for proper names to defer to common pronunciation such as Dickens’s book, Bridges’ film, or Mars’ moons. Mars follows the ancient name rule, and Bridges follows the ends in iz-sound rule.

According to “[Apostrophe Catastrophe (Part 2)]” by Mignon Forgarty aka Grammar Girl, the rule on forming possessive singular of nouns comes down to a style issue. The Associated Press Stylebook recommends leaving off the extra s for proper nouns. The Chicago Manual of Style allows for either way, but prefers adding ‘s.

The different style guides disagree on the exceptions, too. Take a glance at this post on AP vs Chicago (find the contradiction in the examples for bonus points) showing the differences between The Associated Press Stylebook and The Chicago Manual of Style on using apostrophes such as for appearance’ sake vs for appearance’s sake. My flowchart at the bottom covers this and more.

What about a company name that includes an apostrophe? Some style guides don’t cover this, but I think McDonald’s’ (McDonald’s’s) looks strange and sounds silly.

Exception: if common noun ends with s AND next word begins with s, add apostrophe only

  • For goodness’ sake, include comments in your code.
  • The vampire bit into the duchess’ sleeve.
  • Dickens’s story is excellent.

Exception: if proper noun already ends in ‘s then leave it

  • McDonald’s profit climbed six percent last quarter.

Follow your required style guide and be consistent

Journalists, including some bloggers, generally follow The Associated Press Stylebook. Some organizations have their own. The Chicago Manual of Style or Modern Language Association (MLA) are often used among software developers writing journals or textbooks. If you are required to follow a specific style then review the guide, but be aware that different versions of the same guide may disagree with each other.

Flowchart of how to form the possessive of nouns

This handy flowchart illustrates how confusing using apostrophes can become due to different style guides and exceptions. Included in the flowchart are The Chicago Manual of Style (Chicago style), The Associated Press Stylebook (AP style), and Dictionary of Modern English Usage (Fowler’s style). For MLA, follow Chicago style.

"flowchart: forming possessive nouns"

Note that The Chicago Manual of Style has dropped the exception on proper names ending in a z-sound as in Bridges’ film, allowing both styles, but prefers Bridges’s film.

Coders, go write software to check your work for correct apostrophe usage.

Flowchart in large PNG