I’m going to focus on the difference between how The Associated Press Stylebook and The Chicago Manual of Style handle possessives for words ending in s or an s sound. In short, is it Carlos’ stylebook or Carlos’s stylebook?

Both AP and Chicago styles take pronunciation into account, handling new syllables formed by back-to-back sibilants in their own way. The style that many of us are accustomed to—simply adding an apostrophe after the s (e.g., moss’ growth) regardless of how the words sound—is a “formerly more common” alternative practice, according to Chicago, one which it does not recommend. But just between you and me, you can use this shoot-from-the-hip style in personal e-mail, where you are also free to forgo capitalization completely. (This may or may not be a test.)

Whereas quote marks can face left or right, apostrophes only face one way.

The punctuation mark that most often gets mixed up with the apostrophe, by my estimation, is the single quotation mark. If smart (or curly) quote marks are toggled on, beware of employing a beginning single quote mark (6-shaped) to do the job of an apostrophe (9-shaped): Whereas quote marks can face left or right, apostrophes only face one way. (Tip: Remember the mark in don’t, or think of a backwards c.) Prepare to battle text-editing software which defaults to a beginning single quote mark when you begin a paragraph with an apostrophe or key it in after a space, such as for ’80s, ’tis, ’cause, or rock ’n’ roll (apostrophes, all of them).

General Rules for Forming Possessives

Plural Common Nouns Ending in S

AP and Chicago: Add an apostrophe—jinx!

• the students’ questions
• the teachers’ headaches

Singular Common Nouns Ending in S

AP: Add apostrophe-s unless the next word begins with s.

• the duchess’s hat
• the duchess’ style

Chicago: Add apostrophe-s.

• the duchess’s hat
• the duchess’s style

Proper Nouns Ending in S

AP: Add an apostrophe.

• Charlaine Harris’ books
• the Joneses’ competition

Chicago: Add apostrophe-s if singular, and add an apostrophe if plural.

• Socrates’s tea
• the Obamas’ garden
• Les’s moor

Nouns Plural in Form, Singular in Meaning

AP and Chicago: Add an apostrophe.

• the series’ actors
• the Academy of Motion Picture Arts and Sciences’ history

Special Case: Nouns Ending in an Unpronounced S

AP: Add an apostrophe.

• Descartes’ thoughts

Chicago: Add apostrophe-s.

• Camus’s existence
• the debris’s cloud

Special Case: Singular Common Nouns Ending in S or an S Sound, Followed by a Word Beginning With S

AP: Add apostrophe.

• for appearance’ sake
• for conscience’ sake
• for goodness’ sake

Chicago: Add an apostrophe if the word ends in s; otherwise, add apostrophe-s.

• for appearance’s sake
• for conscience’s sake
• for goodness’ sake

Proper nouns ending in s follow previously stated styles (e.g., for Jesus’s sake in Chicago style).

Exception: Company Names With Apostrophe-S

AP: Use as is.

• McDonald’s profits (not McDonald’s’ profits)

So, to answer the question posed in the beginning (Carlos’ stylebook or Carlos’s stylebook?), the first is in AP style, the second is in Chicago style. Let’s hope that Carlos picked the right stylebook.

Sources

• AP, 2011: “apostrophe”; “Ask the Editor”
• CMOS, 16th edition: possessives, 7.15-21