Chicago style checker

Check a history essay, thesis chapter or manuscript against the Chicago Manual of Style, 17th edition: numbers, percentages, dashes, serial commas and author-date citations, each explained with the rule.

APA MLA CMOS AP

Chicago Manual of Style · 17th edition (2017)

The expansion of the railway transformed rural economies across the Midwest during the 19th-century¹. By 1870, more than 50² towns in Illinois depended on a single line (Cronon, 1991, 72)³. Farmers, merchants and bankers⁴ competed for access to freight rates, and many towns failed when the tracks passed them by. As one historian put it, the railroad "made Chicago the hinge of a continental economy" (Cronon 1991, 81). ⁵Contemporary newspapers described the change in dramatic terms; about 40%⁶ of farm income went to transport costs. Ibid.⁷ notes that the effect was uneven -- ⁸some counties prospered while others declined.

91 words · 6 sentences · 25 CMOS rules checked

1errors

4warnings

3notes

1
Century written in numerals
“19th-century”
Chicago and MLA spell out centuries: the nineteenth century.
2
Numeral under 101 in prose
“50” → “fifty”
Chicago’s general rule spells out whole numbers from zero to one hundred in running text.
3
Comma in Chicago author-date citation
“(Cronon, 1991, 72)” → “(Cronon 1991, 72)”
Chicago author-date has no comma between author and year; the page follows a comma: (Ellis 2014, 32).
4
Missing serial (Oxford) comma
“Farmers, merchants and bankers” → “Farmers, merchants, and bankers”
APA, MLA and Chicago all require a comma before the final “and/or” in a series of three or more.
5
Two spaces after punctuation
“. ” → “. ”
All four guides now use one space after a full stop or other punctuation.
6
% symbol in humanities prose
“40%” → “40 percent”
Chicago writes “percent” in running text in humanities work (the symbol is for scientific and statistical copy).
7
“Ibid.” is discouraged
“Ibid.”
Chicago 17 discourages ibid. in favour of shortened notes (Author, Short Title, page).
8
Double hyphen as a dash
“ -- ” → “—”
Use a true em dash (—). APA, MLA and Chicago close it up; AP puts a space on each side.

AI style review

A second read by an AI editor briefed on Chicago Manual of Style: judgment calls the rules can’t make, such as clarity, bias-free wording and citation placement.

Two citation systems, one set of prose rules

Chicago offers notes and bibliography, favoured in history, art history and the humanities, and author-date, favoured in the sciences and social sciences. The prose conventions (how to write numbers, dashes, dates, abbreviations) are the same in both, and those are what the checker enforces in your text.

Author-date citations

Chicago author-date omits the comma between author and year and separates the page with a comma: (Cronon 1991, 72). That single comma is the most common difference from APA’s (Cronon, 1991, p. 72), and the checker flags it.

Footnotes

In notes-bibliography style, a superscript number follows the punctuation, and the first note gives the full source; later notes use a short form: author, short title, page. The 17th edition discourages “ibid.” and the 18th all but retires it. Build notes with the Chicago footnote builder.

Numbers and punctuation

Spell out zero through one hundred in running text (“fifty towns”), write centuries in words (“the nineteenth century”), use “percent” with numerals, close up em dashes with no spaces, and use en dashes for number ranges (72–81).

What this checker looks for

Every CMOS rule the engine applies, grouped as a copy editor would.

Mechanics

Two spaces after punctuation. All four guides now use one space after a full stop or other punctuation.
Space before punctuation. Punctuation attaches to the preceding word in English.
Repeated word. The same word appears twice in a row, usually a typing slip.
Comma after e.g. / i.e.. APA and Chicago follow “e.g.” and “i.e.” with a comma.
Abbreviation missing periods. Write “e.g.” and “i.e.” with periods in US styles.
Hyphen in a number range. Number and page ranges take an en dash (–), not a hyphen.
Double hyphen as a dash. Use a true em dash (—). APA, MLA and Chicago close it up; AP puts a space on each side.
Spaced em dash. APA, MLA and Chicago set the em dash closed, with no spaces.
Ampersand in running text. Spell out “and” in prose; APA reserves & for parenthetical citations and references.

Citations

Malformed “et al.”. “et” is a whole Latin word (no period); “al.” is an abbreviation (period required).
Sentence period before the citation. A parenthetical citation belongs inside the sentence: close the quotation, add the citation, then the period.
“Ibid.” is discouraged. Chicago 17 discourages ibid. in favour of shortened notes (Author, Short Title, page).
Ampersand in citation. MLA and Chicago spell out “and” between authors.
Comma in Chicago author-date citation. Chicago author-date has no comma between author and year; the page follows a comma: (Ellis 2014, 32).

Bias-free language

Binary pronoun pair. APA 7 endorses singular “they”; MLA and Chicago accept it too.
Outdated or biased term. Current style guides recommend neutral, person-first or specific wording.

Punctuation

Missing serial (Oxford) comma. APA, MLA and Chicago all require a comma before the final “and/or” in a series of three or more.

Numbers

Numeral under 101 in prose. Chicago’s general rule spells out whole numbers from zero to one hundred in running text.
% symbol in humanities prose. Chicago writes “percent” in running text in humanities work (the symbol is for scientific and statistical copy).
Century written in numerals. Chicago and MLA spell out centuries: the nineteenth century.

Word choice

British variant. American usage prefers toward, afterward, among, while.

Dates & times

Ordinal suffix in a date. Write March 3, not March 3rd.

Formality

Contraction. Academic styles expect contractions to be written out (do not, it is).
Informal wording. Colloquial phrases read as conversational in academic prose; choose a precise term.
Second person. Research papers rarely address the reader as “you”; rephrase in third person.

Questions

Does the Chicago style checker support footnotes and author-date?

It checks the prose rules that apply to both systems (numbers, percent, centuries, dashes, serial commas, ibid.) and author-date parenthetical format: (Cronon 1991, 72) with no comma before the year. Footnote text itself is best built with the note builder in the Chicago footnotes guide.

Which edition of Chicago does it follow?

The Chicago Manual of Style, 17th edition (2017), with notes where the 18th edition (2024) differs, for example its stronger preference for shortened notes over ibid.

How does Chicago handle numbers?

In general (nontechnical) writing Chicago spells out whole numbers from zero through one hundred and round multiples of those (three thousand), and uses numerals for the rest. Percentages use numerals with the word “percent” in humanities text.

Is Chicago the same as Turabian?

Turabian’s A Manual for Writers is Chicago adapted for student papers. The citation rules match; Turabian adds guidance on title pages and formatting for course papers.

Does Chicago use the Oxford comma?

Yes. Chicago strongly recommends the serial comma before the final “and” or “or” in a series of three or more.