Updated August 12, 2025
Spotlight on Exceptions
Even the most straightforward rule will be subject to an exception sooner or later. That’s why CMOS qualifies so many of its rules with usually or generally. But some exceptions are so common that they deserve to be called rules themselves.
Let’s examine some of the more notable exceptions in terms of the rules they break.
Seven Rules, Eight Exceptions
The following seven rules—and their exceptions—can all be found in CMOS, either explicitly or by example (and sometimes both). To put Chicago’s rules in perspective, some additional exceptions recommended by other guides are also mentioned where relevant, in the explanations following the examples.
1. Do not add an apostrophe to form a plural.
Exception: Individual letters.
Example 1: There are two l’s and two a’s in the word llama.
Example 2: I got A’s in my science classes but B’s in everything else.
Most of us know that it’s two bananas, not two banana’s. But sometimes an apostrophe clarifies a plural that would otherwise be difficult to read.
In Chicago style, letters used as letters usually get italics, but italics alone are too subtle to differentiate a lowercase letter from its plural ending. Compare “two ls and two as” with the first example above; the apostrophes in the example (l’s and a’s) help to clarify that these aren’t the words Is and (especially) as.
Apostrophes can also be helpful with capital letters, where italics aren’t always used, as with letter grades or in the expression “the three R’s.” The meaning of “three Rs” is clear enough without an apostrophe, but what about “two As”?
The pluralizing apostrophe, which had been dropped as a requirement for capital letters in recent editions of CMOS, is once again Chicago style as of the eighteenth edition (see CMOS 7.15).*
Some style guides also specify apostrophes for the plurals of numbers (1920’s) and for abbreviations in all caps (YMCA’s). In Chicago style, that would be 1920s and YMCAs.†
For more on the apostrophe—which is more commonly used in contractions and possessives (when it’s not acting as a single quotation mark)—go to “Chicago Style Workout 65: Apostrophes” and take the quiz.
2. The words in a direct quotation should reflect the source exactly.
Exception: The capitalization of the first letter of quoted text can be adjusted to suit the syntax of the surrounding sentence.
Example: Regarding copyright, the US Constitution gives Congress the power “to promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.”
In the Constitution itself (art. 1, sec. 8), that opening “to” begins with a capital T—but only because it’s the first word in the eighth of eighteen enumerated powers, each of which begins with the word “To.” Outside the context of the original, the capital T has little significance, and Chicago says that it can be adjusted as needed (see CMOS 12.7, rule 3). (The original capitalization of words like “Progress,” “Science,” and “Arts” is more than circumstantial and is therefore retained.)
Some styles say to bracket any such change (i.e., “[t]o promote . . .”; see CMOS 12.21). Those brackets may help readers find the quoted words in the original more quickly, but any advantage from this intervention (which might be required dozens of times in the typical literary or historical study) is too small to justify making it mandatory outside of certain legal and textual studies.
3. Do not begin a sentence or a heading with a lowercase letter.
Exception: Words like iPhone and eBay.
Example: iPhones can always be found on eBay, even if you’re looking for a newer model.
Though some styles say to apply an initial cap to words like iPhone at the beginning of a sentence or heading, such words already feature a capital letter; they don’t need any extra help from the Shift key. See CMOS 8.155.
4. Do not begin a sentence with a numeral.
Exception: Terms that include a mix of letters and digits.
Example: 7-Eleven is known to many as the home of the Slurpee.
Numerals at the beginning of a sentence can be hard to read, especially in a work that features old-style numbers, many of which look like lowercase letters. In the example below, notice how the number 150 is almost hiding at the beginning of the second sentence (whereas the word “Because” stands out as intended):
The initial capital in a term like “7-Eleven” or “3D” makes this less of a problem, as do the parentheses in a term like “401(k).” So for the eighteenth edition we added such terms as exceptions to the usual rule (see CMOS 9.5).
Four-digit years are also usually recognizable at the beginning of a sentence—especially when old-style numerals aren’t being used—so we now allow those also. But we still advise a workaround as the better option: The year 1937 . . .
5. For spelling, follow Merriam-Webster. If an entry lists two or more spellings, choose the first.
Exception: The Chicago Manual of Style spells copyeditor as one word.
The term was first recorded in the Manual as two words, in the index to the twelfth edition: “Copy editor. See Manuscript editor.” But it was spelled as one word in the thirteenth edition (published in 1982), and we’ve never looked back.
Unlike the verb copyedit, which is listed first in Merriam-Webster (ahead of the two-word form copy edit), the noun copyeditor is a “less common” variant (behind the first-listed two-word form copy editor). But we like how the one-word noun copyeditor is consistent with the first-listed verb form—and with the related nouns copyholder, copywriter, and copyreader. See also CMOS 7.1 and 7.2.
6. Abbreviations form the plural by adding s.
Exception 1: Abbreviations for units of measure, which are invariable in both the metric system and the older imperial system—as in 8 km or 3 in. (not 8 kms or 3 ins.).
Exception 2: Irregular plurals like pp. (pages, sing. p.) and MSS (manuscripts, sing. MS) and plurals of abbreviations that already end in s (e.g., trans., which can be used for one translator or more than one translator).
Plurals are always subject to irregularities; abbreviations are no exception. See CMOS 7.15, 10.59, and 10.73 for more details and examples.
7. Each new paragraph in a book gets a first-line indent.
Exception: The first paragraph in a chapter or section.
This is more of a convention than a rule (though CMOS now mentions it; see paragraph 2.15). In the first ten editions of CMOS, judging by the prefaces, first paragraphs were indented—as in the preface to the 1906 first edition:
Here’s the beginning of the preface to the eighteenth edition (in a screenshot from the PDF file used as the basis of the printed book). Note how the first paragraph (but not the second) begins flush left (the intervening epigraph also begins flush left, as most do):
Most books are designed this way now. It’s a nice distinction that shows how the absence of an indent can be almost as meaningful as an indent. Exceptions should always do this. In the context of rules designed to promote consistency and clarity, any departure should be made with the reader in mind.
* * *
For some additional exceptions to established rules, see “When Hyphenation Is Unnecessary” and “Is ‘Is’ Always Capitalized in Titles?” Both of those posts, like this one, have now been updated to reflect the eighteenth edition of CMOS.
* The twelfth edition, which was published in 1969, started this trend by suggesting that the phrase “the three Rs” was clear as is, thereby removing the apostrophe from an example that had been in the Manual since the first edition was published in 1906. We decided for the eighteenth edition that the first edition had been right all along.
† The first eleven editions of CMOS included the examples “1900’s” and “YMCA’s” (or “Y.M.C.A.’s” with periods through the tenth edition). The twelfth edition was the first to drop these apostrophes, a change that (unlike the one for capital letters) has stood the test of time.
Exception and rule sign by Thomas Reimer / Adobe Stock.
Please see our commenting policy.
