Abbreviations

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.

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.

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.

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

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.

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.

A. See CMOS 10.3: “The abbreviation usually follows immediately, in parentheses, but it may be introduced in other ways.” Your approach seems like a sensible and reader-friendly alternative to introducing the abbreviation immediately after the first mention.

A. You could spell out the pronunciation of SQL at the first opportunity in the text—for example, “this data is stored on a SQL (pronounced ‘sequel’) server” (see also CMOS 10.3). Those who are unfamiliar with this pronunciation (from “Structured English Query Language,” or SEQUEL, the name first proposed in the early 1970s) will now be clued in; those who already say “sequel” will have their preference confirmed. But it should be noted that according to ANSI (the American National Standards Institute), the pronunciation of SQL is not a settled issue, and “ess-cue-el” is considered a legitimate option. So it’s not a bad idea to signal a preference regardless of your choice.

A note on the example: Though “data” is usually plural in scientific contexts—“these data are”—“data” is often used as a mass noun in computer-related writing. For example, this usage is allowed by the latest style guides published by Microsoft and Apple.

A. Q&A is a pair of initialisms joined by an ampersand; as such, it’s an abbreviated form of the abbreviated expression “Q and A.” And because “Q and A” is widely known (and has its own entry in Merriam-Webster), there is no need to spell out either form the first time you use it. For a discussion of acronyms and initialisms (both of which are types of abbreviations), see CMOS 10.2; for the absence of spaces in Q&A, see CMOS 10.10.

A. It most likely stands for “plate”—as in an illustration printed on special paper and bound together in a separate section known as a gallery; these pages typically aren’t paginated with the rest of the book, so plate or figure numbers must be used instead of page numbers to refer to individual pages in the gallery. CMOS 10.42 includes nearly 250 abbreviations that might appear in scholarly publications, including “pl.” As you will see in that list, the abbreviation “pl.” can mean either “plate” or “plural” but is best avoided for the former. Whoever recorded “pl.” in the example you cite apparently didn’t see our list.