Chicago Notes–Bibliography vs Author–Date: The Two Chicago Systems Compared
The Chicago Manual of Style is two citation systems sharing one manual. Notes–bibliography uses superscript footnote numbers in the text, full footnotes at the bottom of the page, and a Bibliography at the end of the paper. Author–date uses parenthetical (Author Year) citations in the text and a References list at the end. The two are not interchangeable — the choice between them is determined by discipline before you start writing — and confusing them produces a paper that does neither well. This guide compares the two systems side by side and clarifies which one your assignment most likely expects.
The shortest answer: notes–bibliography for humanities (history, art history, religion, philosophy); author–date for sciences and social sciences that use Chicago. Pick one and apply it consistently — they are not mix-and-match.
What each system is for
Notes–bibliography is the traditional Chicago system, descended from the 1906 first edition of the Manual. Citations appear as numbered footnotes at the bottom of the page (or as endnotes at the end of the chapter), and a Bibliography at the end of the paper lists all sources cited plus any consulted in research. The system is used by history journals, art history journals, religion journals, philosophy journals, and most humanities work that prefers footnotes to parentheticals.
Author–date is the newer Chicago system, added to the Manual in the 1970s when more disciplines adopted the parenthetical style that APA had popularized. Citations appear as (Author Year) in the text, and a References list at the end alphabetizes the sources. The system is used by some economics journals, some political-science journals, and any author who is required to use Chicago but writes in a tradition that expects in-text citations rather than footnotes.
If your discipline is in the humanities, notes–bibliography is your default. If your discipline is in the sciences or social sciences but Chicago is required (rather than APA or another disciplinary default), author–date is your default. If your assignment or journal specifies otherwise, follow what is specified — the Manual permits either.
In-text citation mechanics
The most visible difference. Same single-author source — Chen’s 2021 book The Architecture of Working Memory, page 47.
| Notes–bibliography | Author–date | |
|---|---|---|
| First citation | ¹ (superscript pointing to footnote 1) | (Chen 2021, 47) |
| Subsequent citation | ⁵ (superscript pointing to footnote 5) | (Chen 2021, 92) |
| Narrative | Chen argues that … ³ | Chen (2021, 47) argues that … |
| Quote | A “consolidation effect”⁴ … | … “consolidation effect” (Chen 2021, 47) |
In notes–bibliography, the in-text marker is just a number. The actual citation content lives in the footnote. In author–date, the citation content is in the parenthetical itself, and there are no footnotes.
This shapes how the systems read. A notes–bibliography page can carry dense citation traffic without interrupting the prose — five footnote numbers in a paragraph add five superscripts. The same paragraph in author–date carries five parentheticals, each disclosing the author and year inline. Humanities readers tend to find the first arrangement easier to follow; sciences readers tend to prefer the second because the year is always visible.
The footnote itself
In notes–bibliography, the first citation of any source carries a full footnote. Subsequent citations use a shortened form.
First footnote: 1. Margaret S. Chen, The Architecture of Working Memory (Cambridge: Cambridge University Press, 2021), 47.
Subsequent footnote: 8. Chen, Architecture of Working Memory, 89.
The first footnote contains author (in normal order — first name first), title, publication information, and the specific page. The subsequent footnote contains author surname only, a shortened version of the title, and the page. The shortened title omits initial articles (The, A, An) and uses just enough of the title to identify the source unambiguously.
The Chicago Manual of Style 18th edition (2024) discourages the older Ibid. shortcut — once standard for referring back to the immediately preceding citation — in favor of the shortened-citation form above. The reasoning is practical: Ibid. is fragile during revision (insert a new citation between two Ibid. references and the second one now refers to the wrong source), while the shortened form remains accurate regardless of what surrounds it.
Bibliography versus References
Both systems have a back-of-paper source list. They use different names and slightly different formats.
The same book in each system’s back-matter format:
Notes–bibliography (Bibliography entry): Chen, Margaret S. The Architecture of Working Memory. Cambridge: Cambridge University Press, 2021.
Author–date (References entry): Chen, Margaret S. 2021. The Architecture of Working Memory. Cambridge: Cambridge University Press.
Three differences:
- Year placement. Notes–bibliography places the year at the end of the entry, after the publisher. Author–date places it immediately after the inverted author name.
- Punctuation between elements. Notes–bibliography separates author, title, and publication info with periods. Author–date does the same but inserts the year between author and title, which slightly changes the rhythm.
- What the section is called. Notes–bibliography uses “Bibliography.” Author–date uses “References.”
The Bibliography in notes–bibliography may include both cited and consulted sources — works the writer read for background but did not directly cite. The References list in author–date is restricted to sources actually cited in the body, like APA’s References or MLA’s Works Cited. This is the meaningful difference between the two labels and one of the few cases where the system name is a substantive content rule, not just a labeling choice.
When to pick which
The disciplinary defaults are clear enough that most papers do not face a choice. When they do:
Pick notes–bibliography when:
- Your paper is in history, art history, religion, philosophy, or another humanities field.
- Your prose carries extensive discursive material that footnotes can absorb (qualifications, asides, brief discussions of secondary sources).
- Your readers expect to see citations at the bottom of the page rather than interrupting the prose.
- Your journal or instructor specifies it.
Pick author–date when:
- Your paper is in a science or social-science field that has adopted Chicago rather than APA.
- Your readers expect to see the year of publication on every citation (which signals currency of evidence).
- Your prose is short on discursive footnote material and would benefit from the cleaner running text.
- Your journal or instructor specifies it.
When in doubt, look at the journals your discipline publishes in. If they use footnotes, your paper should use notes–bibliography. If they use parentheticals, your paper should use author–date.
Common cross-system mistakes
Mixing footnotes with parentheticals. A paper that uses footnotes for some citations and (Author Year) for others is in neither system. Pick one before you start drafting.
Using “Bibliography” as the heading on author–date papers. Author–date uses “References.” Using “Bibliography” suggests you set out to write notes–bibliography and forgot to commit to one system.
Using Ibid. for repeated citations. Chicago Manual 18 prefers the shortened-citation form. Ibid. still appears in older Chicago papers and remains permitted, but the current convention is the surname-plus-short-title form because it survives revisions that move text around.
Inverting the author’s name in a footnote. Footnotes use normal name order: “Margaret S. Chen,” not “Chen, Margaret S.” The bibliography entry uses the inverted form (“Chen, Margaret S.”) because that is what supports alphabetization. Mixing the two looks like a draft that was reformatted halfway.
Forgetting to switch to the shortened form after the first footnote. Every footnote after the first reference to a source should use the surname-plus-short-title form. Repeating the full footnote each time is correct in some older styles but is treated as a mistake in current Chicago.
Writing parenthetical citations without the year. Some students transfer the MLA habit of (Chen 47) to Chicago author–date. Chicago author–date requires the year: (Chen 2021, 47). The page locator follows a comma; there is no p. abbreviation.