Q. CMOS omits periods after any designator for United States organizations of any kind—e.g., US Army, US Navy, US Department of State. This format directly contradicts all official U.S. government writing guidelines. What prompted CMOS to make such a change?

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.