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Chicago 17 vs 18: What Changed and How to Update Your Formatting

TL;DR

  • Chicago 18 dropped the publisher city requirement — your book citations won’t need “New York:” or “London:” anymore
  • Author limits changed — up to 6 authors in bibliography, up to 2 in notes/text, then “et al.” kicks in
  • Singular “they” is now endorsed — resolving decades of debate in style guides
  • Turabian 9th still follows Chicago 17 — thousands of students are using outdated rules right now
  • The 18th edition adds section 14.112 for citing AI-generated content — new for tools that didn’t exist when Chicago 17 was published

Introduction

You’ve probably been using Chicago style for a semester or two. Your professor handed down a citation guide. Your citation manager has a template. Everything was working. Then you hit Chicago 18 and suddenly your bibliography looks wrong. Or your footnotes use a form that your professor hasn’t seen yet. Or you get marked down for something you didn’t even notice.

This is exactly what happens when a style guide updates. The Chicago Manual of Style 18th edition, published by the University of Chicago Press in September 2024, is the most comprehensive revision in two decades. It changes citation formatting, grammar rules, and inclusive language guidelines — not in a way that breaks everything, but in enough places that your old Chicago 17 citations won’t match what a Chicago 18 reader expects.

This article covers every substantive change from Chicago 17 to Chicago 18, with concrete before-and-after examples for each format. By the end, you’ll know exactly which citations to update, which to leave alone, and which edition you should actually be using. If you want a broader primer on Chicago style before diving into the differences, our Chicago Style Citation Guide covers all the basics from the ground up. For a quick comparison across all major styles, see our Citation Styles Compared guide.

The Big-Picture Changes

Three categories of changes make up the 18th edition:

1. Citation and bibliography formatting — The publisher city is gone. Chapter page ranges are optional. Author listing thresholds changed. URLs keep trailing slashes. A whole new section covers AI content.

2. Grammar and punctuation — Singular “they” is endorsed. Capitalization after colons changed. Title case now treats longer prepositions differently. Time zones and digital terms get simpler treatment.

3. Inclusive language — All racial and ethnic terms are capitalized. Acknowledgments move to back matter. Emoji guidance enters the manual for the first time.

Most students notice the publisher city first, because it affects every book citation. But the author limit changes are harder to catch, and they’re the ones that actually matter when your bibliography has dozens of sources. Here’s why.

The publisher city drop is the most visible change but not the most consequential. Your forty book citations will look different on the page. But the author limit changes are harder to catch because a citation with the wrong number of names still looks perfectly normal — the visual difference is subtle enough that students won’t notice until a professor marks them down. That’s why students who learned Chicago from 17th-edition guides need targeted attention on et al. limits, not just the publisher-city change.

Citation and Bibliography Changes

This section is where most students get marked down. The changes are specific enough that they don’t trigger your citation manager’s auto-updates, and vague enough that you won’t notice them until a professor asks “why is this formatted differently?”

Publisher City (CMOS 14.30)

The 18th edition removes the publisher city requirement from book citations entirely. This was the single most visible formatting change across the entire edition.

Chicago 17 (Before):

Pollan, Michael. The Omnivore’s Dilemma: A Natural History of Four Meals. New York: Penguin, 2006.

Chicago 18 (After):

Pollan, Michael. The Omnivore’s Dilemma: A Natural History of Four Meals. Penguin, 2006.

Chapter Page Range (CMOS 14.8)

Book chapter page ranges are no longer required in bibliography entries. If the publisher doesn’t provide them, you don’t need to insert them.

Chicago 17 (Before):

La Barbera, MariaCaterina. “Intersectional-Gender and the Locationality of Women ‘in Transit’.” In Feminism and Migration: Cross-Cultural Engagements, edited by Glenda Tibe Bonifacio, 17-31. Amsterdam: Springer, 2012.

Chicago 18 (After):

La Barbera, MariaCaterina. “Intersectional-Gender and the Locationality of Women ‘in Transit’.” In Feminism and Migration: Cross-Cultural Engagements, edited by Glenda Tibe Bonifacio. Amsterdam: Springer, 2012.

Authors in Bibliography (CMOS 13.23)

Up to 6 authors are listed in the bibliography. If there are more than 6, list the first 3, then add “et al.” This is the change students miss because a citation with 4 or 5 names “looks normal” regardless of the correct count.

Chicago 17 (Before):

Up to 10 authors listed, then et al.

Chicago 18 (After):

Up to 6 listed; more than 6: first 3 + et al.

Authors in Notes and In-Text (CMOS 13.23)

The threshold for in-text citations and footnotes dropped from 3 to 2.

Chicago 17 (Before):

Up to 3 authors, then et al.

Chicago 18 (After):

Up to 2 listed; more than 2: first + et al.

Shortened Citations (CMOS 13.37)

The 18th edition gives shortened citations three clear forms and explicitly prefers the author + title form as the regular shape. Ibid. is retired — not discouraged this time, but replaced with three defined alternatives.

Chicago 17 (Before):

Shortened form + ibid. permitted

Chicago 18 (After):

Three forms: author + title (preferred), author only, title only

URLs (CMOS 6.120)

Trailing slashes can now be kept on URLs instead of being dropped.

Chicago 17 (Before):

Trailing slash usually dropped

Chicago 18 (After):

Trailing slash can usually be kept

AI Source Citation (CMOS 14.112)

Section 14.112 provides new guidance for citing AI-generated content — credit the tool as author, record the prompt and date, include the shareable URL when available. This didn’t exist in Chicago 17 because the tools didn’t exist.

Chicago 17 (Before):

No guidance

Chicago 18 (After):

New guidance: credit tool as author, record prompt & date

Full Comparison Table

Change CMOS Section 17th Edition 18th Edition
Publisher city 14.30 Required in book citations No longer required
Chapter page range 14.8 Required in bibliography No longer required
Authors in bibliography 13.23 Up to 10, then et al. Up to 6; >6: first 3 + et al.
Authors in notes/text 13.23 Up to 3, then et al. Up to 2; >2: first + et al.
Shortened citations 13.37 Shortened form + ibid permitted Three forms: author + title (preferred), author only, title only
URLs 6.120 Trailing slash usually dropped Trailing slash can usually be kept
AI source citation 14.112 No guidance Credit tool as author, record prompt & date

The table above is your quick reference. Don’t try to memorize every CMOS section number — focus on the actual formatting change and whether it affects citations you’re already using.

Grammar and Style Changes

Chicago 18 didn’t just change citations. It updated several grammar conventions that students encounter when writing the main text of their papers. These changes are less likely to trigger a formatting mark, but they matter for consistency.

Singular “They”

The 18th edition explicitly endorses singular “they” as a generic and referential pronoun when a person’s gender is unknown or unspecified. This resolves decades of debate that started with the 17th edition’s careful equivocation.

Chicago 17 (Before):

Not explicitly endorsed

Chicago 18 (After):

Explicitly endorsed for generic and referential use

This is the single most significant grammar change in the edition. If a student’s professor requires Chicago 17 formatting, they should follow the older convention. But unless the course specifies, Chicago 18’s endorsement is the default.

Capitalization After Colons

If a complete sentence follows a colon, the first letter is now capitalized.

Chicago 17 (Before):

Sentence case (lowercase after colon)

Chicago 18 (After):

Capitalize the first letter if it starts a complete sentence

Title Case Prepositions

Prepositions with five or more letters are now capitalized in title case. Prepositions of four letters or fewer remain lowercase.

Chicago 17 (Before):

All prepositions lowercase regardless of length

Chicago 18 (After):

Capitalize prepositions with 5+ letters (about, beyond, throughout, without); 4 letters or fewer remain lowercase

Time Zones and Digital Terms

Time zone names are lowercased. Hyphens are dropped from words like “ebook” and “esports.”

Chicago 17 (Before):

Time zones capitalized (Eastern Standard Time); hyphenated (ebook, esports)

Chicago 18 (After):

Time zones lowercased (eastern standard time); hyphen dropped (ebook, esports)

Capitalizing Racial and Ethnic Terms

All racial and ethnic terms are now capitalized uniformly. This includes “Black,” “Indigenous,” “Native,” and similar terms used as nouns or modifiers.

Chicago 17 (Before):

Some capitalized (e.g., “Black”)

Chicago 18 (After):

All racial and ethnic terms capitalized

End Dashes for Compound Names

End dashes are now endorsed for open-compound proper adjectives and nouns.

Chicago 17 (Before):

Not explicitly endorsed

Chicago 18 (After):

Endorsed for open-compound proper adjectives/nouns

Which Edition Should You Use?

Here’s the practical question most students actually need answered. You’re not writing a style guide. You’re writing a paper. Which edition does your professor expect?

The decision table below covers the most common scenarios.

Edition Decision Table

Your Situation What You Should Use Why
Professor specifies “Chicago 17” Chicago 17 They’re grading against a 17th-edition rubric
Professor specifies “Chicago Manual of Style” (no edition) Chicago 18 It’s the current manual; this is the default
Professor specifies “Turabian” (no edition) Check current class materials — likely Turabian 9th or 10th Turabian 9 follows Chicago 17; Turabian 10 follows Chicago 18
Writing in history or literature Notes & Bibliography system (NB) This is the Chicago default for humanities
Writing in social sciences Author-Date system (A-D) Social sciences prioritize recency over archival detail
Writing in physical sciences Author-Date system (A-D) Same logic as social sciences
Your journal requires Chicago Check the journal’s submission guidelines Journals sometimes specify edition or system
Course uses Turabian 9th Chicago 17 formatting rules Turabian 9 still follows Chicago 17
Your class uses Turabian 10th (anticipated Oct 2026) Chicago 18 formatting rules Turabian 10 is expected to align with Chicago 18

The most common mistake students make: using Turabian 9th rules when Chicago 18 is required. Thousands of student papers use publisher cities that Chicago 18 no longer requires. The inconsistency is real and noticeable to anyone grading carefully.

If your professor didn’t specify an edition, use Chicago 18. If they did, follow their instructions. When in doubt, ask — a two-minute email saves hours of reformatting.

How to Update Your Citations

You don’t need to reformat every citation from scratch. Here’s the practical sequence:

1. Check your professor’s edition requirement — This determines whether you update anything at all. If they want Chicago 17, your citations are already correct.

2. Run a publisher city sweep — Search for “New York:” or “London:” or other publisher cities in your bibliography. Remove them from every book citation. This is the easiest and fastest update.

3. Check author lists — For each bibliography entry with more than 6 authors, cut it down to 3, then add “et al.” For footnotes and in-text citations with more than 2 authors, keep only 1, then add “et al.” This catches the subtle error most students miss.

4. Replace ibid. — If you use footnotes, replace every instance of ibid. with the preferred shortened form: author + title + page number. Three forms only.

5. Verify URLs — Add trailing slashes back if you previously removed them. The 18th edition says they can stay.

6. Add AI citations — If you used AI tools, follow section 14.112: credit the tool, record the prompt, record the date, include the shareable URL. This didn’t exist in Chicago 17, so you’ll be adding these regardless. For more detail on Chicago’s AI citation guidance, see our How to Cite AI Chat Conversations guide.

7. Check singular “they” usage — If your paper uses gender-neutral pronouns, singular “they” is now the endorsed default. No revision needed unless your professor requires Chicago 17.

8. Handle special source types — Chicago 18 has updated guidance for images, YouTube videos, and social media. If your paper cites any of these, check the updated formats. See our guides on citing images, YouTube videos, and social media for Chicago-specific formatting examples.

Summary and Next Steps

Chicago 18 introduced changes across three categories: citations, grammar, and inclusive language. The publisher city drop is the most visible update. The author limit shifts are the most consequential. Singular “they” endorsement resolves a decades-long debate. Turabian students should check their edition — if you’re on Turabian 9th, you’re using Chicago 17 rules by default.

Bottom line: If your professor didn’t specify an edition, use Chicago 18. Update your bibliography for publisher cities, author limits, and shortened citations. If they specified Chicago 17 or a course manual, follow their version. The edition matters less than consistency — but mixing editions within one paper signals you don’t know the difference.

FAQ

What is the main difference between Chicago 17 and 18?

The 18th edition removed the publisher city requirement, updated author listing thresholds (6 in bibliography, 2 in notes), replaced ibid. with three shortened forms, and added section 14.112 for citing AI-generated content. Grammar changes include singular “they” endorsement and new capitalization rules.

Should I update my citations from Chicago 17 to 18?

Only if your professor expects Chicago 18. If they specified an edition, use that version. If no edition was specified, use Chicago 18 — it’s the current manual. Check whether your bibliography has publisher cities, outdated author lists, or ibid. entries that need updating.

Is Turabian 10th edition released yet?

As of October 2026, the upcoming Turabian 10th edition is expected. It will align with Chicago 18. Students currently using Turabian 9th are still on Chicago 17 formatting rules.

What do I do if my professor didn’t specify an edition?

Use Chicago 18. It’s the current manual published by the University of Chicago Press in September 2024. Unless your assignment explicitly references the 17th edition, Chicago 18 is the default.

What’s the new section 14.112 about?

Section 14.112 provides guidance for citing AI-generated content: credit the tool as author, record the prompt and date, include the shareable URL when available. Chicago treats AI chats as personal communication — cite in footnotes only, not in the bibliography.

How do I know which Chicago system to use?

Notes and Bibliography (footnotes) is the default for history, literature, philosophy, and the arts. Author-Date (parenthetical) is preferred in the social sciences and physical sciences. Your professor or journal may override this.

Sources & Further Reading