---
name: chicago-style
description: Apply Chicago Manual of Style (CMOS) conventions when editing or drafting long-form, literary, academic, or book-length writing — novels, manuscripts, formal reports, book proposals, or any piece using footnotes/endnotes and a bibliography. Use this whenever the user asks to "Chicago style" something, "check Chicago style," or is working on long-form fiction or nonfiction manuscripts (including Julius) without specifying a different style guide. Do not use for press releases, news-style web copy, or short marketing pieces — use the ap-style skill instead in those cases. If the user hasn't said which style they want, ask before applying one.
---

# Chicago Manual of Style Editor

Applies commonly known Chicago Manual of Style (CMOS) conventions to a piece of writing. This skill is built from widely taught, publicly known CMOS rules — it does not reproduce text from the Manual itself. For an edge case not covered here, or a definitive ruling, tell the user to check the current edition (18th, as of this skill's creation) or the online Chicago Manual of Style Online subscription, since specific rulings can be nuanced and this skill won't capture every case.

## Core rules to apply

**Numbers**
- Spell out zero through one hundred, and round multiples of those numbers (two hundred, three thousand); use numerals above that.
- Exception: use numerals for percentages, dates, addresses, and precise measurements even under 100.
- Spell out a number that begins a sentence, or rewrite the sentence.

**Punctuation**
- Use the serial (Oxford) comma: "red, white, and blue."
- Em dashes with no spaces on either side for a break in thought: "the plan—if it worked—would save them."
- Use the em dash sparingly and only where it genuinely clarifies structure; don't default to it as a substitute for a period, comma, or parentheses.
- Commas and periods go inside closing quotation marks; colons and semicolons go outside.
- Use a comma before "too" only when a strong pause is intended.

**Dates and times**
- Spell out full dates: "January 5, 2027," with a comma after the day and after the year mid-sentence.
- Spell out times in narrative prose: "nine o'clock," "half past three," rather than "9:00 a.m.," unless precision matters.

**Titles**
- Use title case (headline-style capitalization) for book, chapter, and article titles: capitalize principal words, lowercase articles/short prepositions/conjunctions unless they start or end the title.
- Italicize titles of full-length works (books, plays, films, albums); use quotation marks for shorter works (chapters, articles, poems, songs).

**Capitalization**
- Lowercase job titles unless immediately preceding a name as a formal title, similar to AP, though CMOS is generally more permissive about capitalizing formal titles in narrative contexts — apply judgment based on register.
- Capitalize proper nouns; lowercase common nouns and general references.

**Dialogue and prose mechanics (fiction-specific)**
- New paragraph for each new speaker in dialogue.
- Use a comma (not a period) before a dialogue tag: "I don't know," she said. — not "I don't know." She said.
- Action beats without a speech verb get their own sentence, not a comma: "I don't know." She looked away.
- Ellipses for trailing off: three periods with spaces per Chicago's traditional spacing convention (a single ellipsis character is acceptable in most modern manuscript contexts — flag this as a formatting choice rather than a hard rule).

**Possessives**
- Add 's to singular nouns ending in s: "Chris's book," per Chicago's preferred rule (this