Abbreviations

A. Tables can be a challenge to format; abbreviations are generally useful there. And in footnotes and bibliographies and other places where sources are cited, abbreviations are customary for certain commonly used words, though Chicago doesn’t require them for months (some styles, however, do). Outside of those contexts, abbreviations are usually unnecessary—that is, until you run out of space (e.g., in the running heads for a printed book).

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. The advice in CMOS to write, for example, “10%” rather than “10 %” (see paragraph 9.18) reflects how people generally use the percent symbol in English-language documents across a wide variety of genres, from fiction to technical reports. (But note that Chicago style for nontechnical settings is to spell it out: “10 percent.”) You’ll find this preference in the AP Stylebook (journalism), the AMA Manual of Style (medicine), and Scientific Style and Format (general sciences), among other guides.

Meanwhile, we are aware that a space is used between a numeral and the percent symbol in the International System of Units (SI), which is official throughout much of the world (see SI Brochure, 9th ed., 5.4.7, where this rule is stated). See CMOS 10.58, where we acknowledge this usage.

The discrepancy between SI, on the one hand, and Chicago and other styles, on the other, may simply be a matter of common usage versus usage in the sciences. It’s worth noting, however, that the SI Brochure, published by the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, or BIPM), is primarily a French-language document; though it’s published simultaneously in English, the French text remains official (see SI Brochure, p. 124).

In French, spaces are used before certain marks of punctuation, including colons and semicolons, and between quotation marks (or, in French usage, guillemets) and the text they enclose. And though these conventions aren’t followed in the English version, perhaps they have something to do with the persistence of certain other spaces in the brochure—for example, the one in an expression like “5 °C,” which is commonly rendered in nontechnical contexts without a space.

Editors following Chicago style would support a strict interpretation of SI style in appropriate contexts—or query its use before making any changes. But we don’t all have to write like international scientists (as admirable as such a goal might be), nor does the space carry any special meaning in the sciences or elsewhere. As conventions go, this one is 100% (or 100 %) arbitrary, provided you’ve been consistent.

[Editor’s note: As a reader has kindly pointed out, a space between a number and a percent symbol or other unit represents multiplication. Our original point was simply that (for example) 100% and 100 % are generally intended to mean the same thing.]

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. If you mean capital letters T and F without periods, then yes. Chicago style requires periods with initials in names—as in “F. Scott Fitzgerald” and “T. S. Eliot”—but not with other types of capitalized initialisms (most of which include at least two letters, as in “US”; see CMOS 10.4). And though the abbreviations for t defined as “true” and for f defined as “false” are lowercase as main entries in Merriam-Webster—where other meanings range from “metric ton” to “folio” and “full”—a “Kids Definition” farther down the page for each entry capitalizes T and F to mean “true” and “false,” suggesting their common use in quizzes and tests. For spelling out abbreviations on first use, see CMOS 10.3.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. There are six ways to write the abbreviations for ante meridiem (before noon) and post meridiem (after noon):

All caps with periods: 10 A.M., 10 P.M.
All caps without periods: 10 AM, 10 PM
Small caps with periods: 10 A.M., 10 P.M.
Small caps without periods: 10 AM, 10 PM
Lowercase with periods: 10 a.m., 10 p.m.
Lowercase without periods: 10 am, 10 pm

Each of these—including “am” and “pm”—is a legitimate choice. For nearly a century, Chicago’s preferred form was the third: small capital letters with periods. This preference, however, applied only to published documents (among other factors, small capitals weren’t an option on typewriters).

When we changed our preference to “a.m.” and “p.m.” (in 2003, with the publication of CMOS 15), the growth of computers in writing and publishing played a role: small caps require extra steps to apply, and they don’t always translate well across applications (when they’re even available). We could have flipped a coin and settled on all-caps “AM” and “PM” (but not “A.M.” and “P.M.”; Chicago style now omits periods in abbreviations that include two or more capital letters). When we instead chose lowercase “a.m.” and “p.m.,” we liked the fact that they’re unambiguous (“AM” and “PM” both have a number of other meanings), and we hoped the periods would help readers recognize in any context that these are abbreviations, not words.

But if you don’t like the periods, don’t fret: Merriam-Webster labels “am” and “pm” as British variants, so you’re hardly alone in your preference. If you’re being published, however, be prepared to defer to your publisher’s house style, whatever that may be.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. You’ve identified two commonly used conventions in finance, one derived from Greek and the other from Latin, but neither one is standard.

Starting with the second convention, M is used for amounts in the thousands and MM for amounts in the millions (usually without a space between the number and the abbreviation—e.g., $150M for $150,000 and $150MM for $150 million). This convention overlaps with the conventions for writing roman numerals, according to which a thousand is represented by M (from mille, the Latin word for “thousand”). Any similarity with roman numerals ends there, however, because MM in roman numerals means two thousand, not a thousand thousands, or one million, as in financial contexts (the year 2020 in roman numerals is MMXX). Likewise, MMM in roman numerals means three thousand, not a thousand times a thousand times a thousand, or one billion. (For an overview of roman numerals, see CMOS 9.65–67.)

According to the other convention, K is used instead of M for thousands (as in $150K for $150,000), because K stands for kilo, the Greek-derived term often used as a prefix to mean “thousand.” This meaning is standard in the sciences, where the kilogram (abbreviated “kg” and equal to 1,000 grams) is the base unit of mass (see CMOS 10.54). K is also used in computing to mean “kilobyte,” but mostly in commercial contexts as a shortening of KB (see CMOS 10.49). To further muddy the waters, K can also mean “kelvin,” which is the base unit of temperature.

Nonetheless, if K is used for thousands, then according to the same convention M (mega) would be used for millions, and billions would be represented by G (giga). If B is typically used instead of G, the reason is obvious: even if you don’t know anything, you might guess that B means “billion.” G, on the other hand, is a slangy shorthand for “grand,” as in a thousand dollars, which might disqualify it as an abbreviation for billions in financial contexts. (Nor is the meaning of “billion” itself an entirely settled matter; see CMOS 9.8.)

So we’re back to square one. Unlike the conventions in science, which are universal (assuming you adhere to the international system of units, or SI), the conventions in finance vary, not only by country but also among institutions, even within the same country. For that reason, in financial contexts it’s best to define up front the convention you are using—whether it’s M and MM and MMM or K and M and B (or G) or something else—to make sure your readers are on the same page. See also CMOS 9.24.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. CMOS dropped the periods in “US”—first as an option (15th ed., 2003) and then as a preference (16th ed., 2010)—in order to move toward what we think is a more logical, streamlined approach to abbreviations. According to this logic, periods are omitted from abbreviations in all caps (like “US” or “NASA”) but not from abbreviations that end in a lowercase letter (like “a.m.” and “p.m.” or “etc.” or “Dr.”). These principles are outlined in CMOS 10.4.

There are exceptions to the lowercase rule—for example, periods are never used in “kg” and “Kbps” and “mph” and many other units of measure (see CMOS 10.49).

In preferring “US” to “U.S.,” it is true that CMOS not only breaks with tradition but also contravenes the recommendations in the GPO Style Manual (the guide for the US Government Publishing Office). But to follow GPO style, you must learn a different set of rules—according to which, for example, “C.P.A.” (certified public accountant) takes periods but “CPI” (consumer price index) does not. Nor does CMOS align with the journalistic recommendations of the Associated Press, according to which “U.S.” and “U.K.” take periods (except in headlines) but “EU” and “AP” do not.

Each style has its own logic and its own set of traditions, tailored to specific types of writing and specific audiences, and Chicago is no different in being different from the others.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. The abbreviation AD stands for anno Domini, Latin for “in the year of the Lord,” so the order AD 99 is required for proper syntax: “in the year 99” makes sense; “99 in the year” does not. BC, which means “before Christ” (the English-language complement to AD), naturally follows the year: 99 [years] BC. The problem with AD and centuries is that they don’t mix: “in the year of the Lord first century” and “first century in the year of the Lord” both fail the test. Not to worry. English and Latin were never meant to coexist, and “the first century AD” is a perfectly acceptable way of referring to “the first century after the year 1 BC” (there is no year zero). (The alternative, “the AD first century,” doesn’t quite work.) Some writers use CE ([of the] Common Era) and BCE (before the Common Era), both of which follow the year, but the older abbreviations have persisted and are more likely to be understood by readers. See CMOS 9.34 for some additional considerations.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. We prefer all-caps “COVID-19.” To find out why, please see “Styling COVID-19 and Related Terms” at CMOS Shop Talk.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. Having an MBA in business administration does sound a little like using a PIN number at an ATM machine to get money to repair your broken LCD display so you can read about RAS syndrome on Wikipedia over a cup of chai tea. But if your colleague has an MBA in marketing or finance, you may want to make your own concentration in business administration explicit. It’s probably best, therefore, to ask before editing out the apparent redundancy.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]

A. Chicago style for the abbreviation of “postscript” can be deduced from our recommendations for using periods with abbreviations (CMOS 10.4), which can be summarized as follows: (a) use periods for an abbreviation that consists of lowercase letters or that ends in a lowercase letter, such as p. or pp., a.m. or p.m., and Dr.; (b) use a period for an initial standing in for a name, as in E. B. White; but (c) omit periods from abbreviations that include two or more capital letters, such as US, PhD, and CEO.

So we would advise writing “PS,” no periods. (The lowercase alternative, “p.s.,” doesn’t seem to be supported by tradition. And the glossary entry you refer to is for the abbreviation of PostScript, the specialized programming language from Adobe—so that doesn’t count.)

As for our own use of “P.S.”—with periods—guilty as charged. As the Q&A has developed over the last twenty-plus years, Chicago style has evolved. Until 2003, we would have advised periods, but then we dropped them—first from academic degrees and most other abbreviations with capital letters (in the 15th ed.), and then also from “U.S.” (in the 16th ed.).

Another consideration: Postscripts are a little old-fashioned (you can usually go back and edit the body of your letter or email or whatever), and so are periods.

But that’s all in the past. Thanks to your query and others like it, we hereby announce “PS”—no periods—as our preference for “postscript.”

PS: You can follow “PS” with either a colon or a period. With our updated preference, the colon is best, but if you prefer “P.S.” you can leave it out.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]