Chicago Style Citations: Notes and Author-Date Guide

Ask two professors what “Chicago style” means and you might get two different answers – one wants footnotes, the other wants author-date parentheticals. Both are correct, because Chicago is actually two citation systems bound into one manual. Picking the wrong one, or mixing them within a single paper, is a formatting mistake graders notice immediately.

This guide walks through both systems – notes-bibliography and author-date – and shows the same sources formatted in each, so you can see exactly where they overlap and where they diverge.

Two Systems, One Manual

The Chicago Manual of Style (18th edition) documents two separate citation systems:

Notes-Bibliography (NB): Citations appear as footnotes or endnotes, with a bibliography listing full source details at the end. This is the system most humanities departments use – history, art history, philosophy, and literature courses default to it.

Author-Date (AD): Citations appear as parenthetical (Author Year) references in the text, with a reference list at the end. This system is closer to APA and is common in the physical, natural, and social sciences.

CiteTools renders Chicago 18 notes and bibliography. The author-date examples below explain Chicago’s separate system but do not describe CiteTools output.

Your assignment or department almost always specifies which one to use. If it doesn’t, ask – the two systems produce entries that look nothing alike for the same source, and reformatting an entire paper’s citations after the fact is a genuinely painful afternoon.

Chicago is unusual among the major styles in that it doesn’t commit to one philosophy of citation. APA and MLA each enforce a single in-text format across every discipline that uses them. Chicago instead lets the discipline decide: footnotes for fields where a citation might carry an aside worth reading, and author-date for fields where in-text brevity and a scannable reference list matter more than editorial commentary.

Citing a Secondary Source in Chicago 18th Edition

Chicago’s flexibility shows up clearly when you’re citing a source you read about secondhand – something quoted inside another author’s book or article rather than a source you read directly.

Notes-bibliography footnote:

1. Miguel Delgado, “Early Warning Systems in Coastal Communities,” Climate Policy Review 8, no. 1 (2019): 22, quoted in Sofia Renner, “Digital Archives and the Future of Historical Research,” Journal of Digital Humanities 9, no. 2 (2024): 118.

Author-date in-text:

(Delgado 2019, cited in Renner 2024, 118)

Only the source you consulted – Renner – belongs in the final bibliography or reference list. See our secondary sources guide for other styles.

Notes-Bibliography: Three Forms of the Same Citation

The NB system uses three different forms of a citation depending on context, and knowing when to use each one is the core skill.

Full note: The first time you cite a source, the footnote includes complete details – author, title, publication information, and the specific page.

Shortened note: After the full citation, use the author’s last name, the title, and the relevant locator.

Here is the difference in practice:

3. Sofia Renner, “Digital Archives and the Future of Historical Research,” Journal of Digital Humanities 9, no. 2 (2024): 118.
4. Renner, “Digital Archives and the Future of Historical Research,” 121.

Bibliography entry: A separate, alphabetized list at the end of the paper with the fullest version of each source’s details, formatted with the first author’s name inverted.

Example 1: Journal Article in Both Systems

Here’s the same journal article formatted for notes-bibliography and author-date.

Notes-Bibliography

Full footnote:

1. Sofia Renner, “Digital Archives and the Future of Historical Research,” Journal of Digital Humanities 9, no. 2 (2024): 118, https://doi.org/10.1234/example.

Shortened note (subsequent citation):

2. Renner, “Digital Archives and the Future of Historical Research,” 121.

Bibliography entry:

Renner, Sofia. “Digital Archives and the Future of Historical Research.” Journal of Digital Humanities 9, no. 2 (2024): 112–30. https://doi.org/10.1234/example.

Author-Date

In-text citation:

(Renner 2024, 118)

Reference list entry:

Renner, Sofia. 2024. “Digital Archives and the Future of Historical Research.” Journal of Digital Humanities 9 (2): 112–30. https://doi.org/10.1234/example.

The year follows the author’s name in author-date. Notes and bibliography places it with the journal facts.

Example 2: Book in Both Systems

Notes-Bibliography

Full footnote:

1. Julian Okafor, Rebuilding Trust: Institutions After Crisis (University of Chicago Press, 2023), 54.

Shortened note:

2. Okafor, Rebuilding Trust: Institutions After Crisis, 58.

Bibliography entry:

Okafor, Julian. Rebuilding Trust: Institutions After Crisis. University of Chicago Press, 2023.

Author-Date

In-text citation:

(Okafor 2023, 54)

Reference list entry:

Okafor, Julian. 2023. Rebuilding Trust: Institutions After Crisis. University of Chicago Press.

Example 3: Website in Both Systems

Websites and organizational authors follow the same two-system logic.

Notes-Bibliography

Full footnote:

1. World Health Organization, “Global Mental Health Report 2024,” WHO, March 3, 2024, https://www.who.int/mental-health-report-2024.

Shortened note:

2. World Health Organization, “Global Mental Health Report.”

Bibliography entry:

World Health Organization. “Global Mental Health Report 2024.” WHO. March 3, 2024. https://www.who.int/mental-health-report-2024.

Author-Date

In-text citation:

(World Health Organization 2024)

Reference list entry:

World Health Organization. 2024. “Global Mental Health Report 2024.” WHO. March 3, 2024. https://www.who.int/mental-health-report-2024.

Example 4: Book Chapter in Both Systems

A chapter from an edited collection carries an extra name – the editor – alongside the chapter’s own author, and both systems handle that credit differently.

Notes-Bibliography

Full footnote:

1. Elena Marsh, “Citation Habits in the Digital Age,” in Research Methods for the Next Generation, ed. Daniel Osei and Priya Nair (Bloomsbury Academic, 2025), 92.

Shortened note:

2. Marsh, “Citation Habits in the Digital Age,” 96.

Bibliography entry:

Marsh, Elena. “Citation Habits in the Digital Age.” In Research Methods for the Next Generation, edited by Daniel Osei and Priya Nair. Bloomsbury Academic, 2025.

Author-Date

In-text citation:

(Marsh 2025, 92)

Reference list entry:

Marsh, Elena. 2025. “Citation Habits in the Digital Age.” In Research Methods for the Next Generation, edited by Daniel Osei and Priya Nair. Bloomsbury Academic.

Chicago 18 no longer requires the chapter page range in bibliography or reference-list entries. Cite a specific page in the note or text citation.

Multiple Authors in Chicago 18th Edition

Author counts affect notes-bibliography and author-date differently.

Notes and bibliography notes: List both authors. For three or more, use the first author followed by “et al.”:

1. Sofia Renner and Julian Okafor, “Title,” Journal Name 9, no. 2 (2024): 45.
2. Sofia Renner et al., “Title,” Journal Name 9, no. 2 (2024): 45.

Notes and bibliography bibliography: List up to six authors. For more than six, list the first three followed by “et al.”

Author-date: Reference lists use the same six-author threshold. Text citations use the first author followed by “et al.” for three or more authors.

Quick Reference: NB vs. AD at a Glance

Chicago’s own name for these systems can trip students up too – “notes-bibliography” is sometimes shortened to “notes” or “Chicago A,” while “author-date” is sometimes called “Chicago B” in older course materials. If your syllabus uses that lettering, “Chicago A” means notes-bibliography and “Chicago B” means author-date. Neither label appears in the current manual itself, but both show up often enough in departmental style sheets that it’s worth recognizing them on sight rather than assuming your professor invented a third system.

Formatting the Notes and Bibliography Pages

• Footnote numbering: Follow your assignment’s numbering scheme and keep note numbers sequential within each numbered unit.
• Bibliography heading: Center “Bibliography” (notes-bibliography) or “References” (author-date) at the top of the page.
• Alphabetical order: Both the bibliography and the author-date reference list are alphabetized by the first author’s last name.
• Hanging indent: Every entry uses a hanging indent, the same way APA and MLA do.
• Spacing: Footnotes are typically single-spaced with a blank line between them; the bibliography or reference list is usually double-spaced, though some instructors accept single-spacing within entries.

Which System Should You Use?

If you have a choice, base it on your discipline rather than personal preference:

• Choose notes-bibliography for history, art history, literature, philosophy, or any paper where you want footnotes free to carry commentary alongside the citation itself.
• Choose author-date for social sciences, natural sciences, or any paper where in-text brevity matters more than footnote flexibility.

If your course uses MLA instead, our MLA 9th edition guide covers its core elements and container system in the same depth. For a broader comparison of how Chicago fits alongside APA and MLA, see our APA vs MLA vs Chicago guide. If your paper cites a source secondhand – something you read about in another author’s work rather than the original – Chicago has specific rules for that situation too; see our secondary sources guide.

Common Mistakes

1. Mixing footnotes and parenthetical citations in the same paper. Pick one system before you start writing and use it throughout.
2. Using a full note where a shortened note is required. Use the shortened form after the source’s full citation.
3. Forgetting to invert only the first author’s name. In a bibliography or reference list entry with multiple authors, only the first author is listed last-name-first.
4. Leaving the year out of an author-date in-text citation. Unlike MLA, Chicago author-date always includes the year, even in a narrative sentence: “Renner (2024) found that...”
5. Moving the year between systems without changing punctuation. Author-date puts the year after the author. Notes and bibliography keeps it with publication facts.
6. Listing three authors in a note. Chicago 18 uses the first author followed by “et al.” for three or more authors.

For the full text of Chicago’s citation rules, The Chicago Manual of Style is the authoritative source and worth bookmarking if you’ll be using this style regularly.

Try It with CiteTools

Drop a DOI, URL, or ISBN into CiteTools.io and select Chicago. CiteTools currently renders a Chicago 18 notes and bibliography entry.