Five differences between AP Style and Chicago Style

As we mentioned in a previous post about six common AP Style mistakes, while University Relations’ uses AP Style, the Internal Communications team, Marketing team, and others use Chicago Style in communications. There are several differences between the two and we wanted to touch on five of them.

If you ever need guidance on either style, remember that the University pays for subscriptions to both style guides. To access them, visit the Libraries’ website, type the name of the style (AP Style or Chicago Manual of Style) in the search bar, and click the blue text that comes up. Then you log into the guide using your University ID.

Oxford comma

As we touched on in that previous post, one of the biggest differences between the two is that AP Style doesn’t use an Oxford comma, also called the serial comma.

  • AP: The flag is red, white and blue.
  • Chicago: The flag is red, white, and blue.

Em dashes

AP style includes a space on either side of an em dash, while Chicago does not. Again, note the use of the Oxford comma in the Chicago Style example.

  • AP: After excelling in a mix of challenging courses — physics, organic chemistry and physiology — Amy decided to pursue a career in medicine.
  • Chicago: After excelling in a mix of challenging courses—physics, organic chemistry, and physiology—Amy decided to pursue a career in medicine.

Possessive apostrophes with singular proper nouns ending in “s”

In AP style, only add an apostrophe to a singular proper noun ending in “s.” In Chicago Style, add an apostrophe and an “s” at the end of a singular proper noun that ends in an “s.”

  • AP: Playing soccer was James’ favorite pastime.  
  • Chicago: Playing soccer was James’s favorite pastime.


In AP style, only spell out numbers under 10 before switching to numerals. In Chicago Style, spell out numbers 0 to 99. For numbers 100 and up, use numerals.

  • AP: There were 86 attendees at the fundraiser party last night.
  • Chicago: There were eighty-six attendees at the fundraiser party last night.


AP style generally uses quotation marks around titles of significant works while Chicago Style uses italics.

  • AP: “The Color Purple”
  • Chicago: The Color Purple

If you have any questions about AP Style, reach out to the University Relations Public Relations team at, and if you have questions about Chicago Style, reach out to the UR Internal Communications team at