Q. I’m proofreading a manuscript in which US is abbreviated without periods throughout. But when it’s part of a compound, periods are added. (“The U.S.-ratified agreement,” or “U.S.-friendly leaders,” for example.) My impulse is to change it, but it appears so regularly that it seems to have been done this way on purpose. (There are 35 instances over 400 pages of text.) Is there ever a reason to use periods in some instances but not others, when you’re abbreviating “United States”?

Q. I frequently use the acronym GAAP, which stands for generally accepted accounting principles. Would the acronym be considered a collective noun? Would I treat GAAP as a singular subject when using the acronym but as a plural subject when spelling it out? What if I do both (spell it out and then put the acronym in parentheses)?

Q. When, if ever, is it acceptable to use the abbreviation for To Whom It May Concern (TWIMC) in a letter?

Q. I read in one of the questions on the website that it is acceptable to begin a sentence with an acronym. If the acronym is not a common one, NASA for example, wouldn’t it be appropriate to instead begin the sentence with the word The and then the acronym? I am having a difficult time with beginning a sentence with acronyms.

Q. My client for a project that uses CMOS has asked that abbreviations ending with S be pluralized without the addition of a lowercase s. So, for example, a first reference is to “asset-backed securities (ABS)” rather than to “asset-backed securities (ABSs),” and subsequent references use ABS as a stand-in for either the singular or the plural term. I cited CMOS 7.15—but the client is “used to seeing” abbreviations without the added s and says it “looks awkward.” I accept that the client gets to call the shots, so I acceded to the request. Did I accede too readily?

Q. I am inquiring about the use of plural acronyms in parentheses. The writer wrote “A three-person board comprised of Senior Non-Commissioned Officer (SNCO)s.” Their use of “(SNCO)s” just looks strange. I said it should be “Senior Non-Commissioned Officers (SNCOs).”

Q. The abbreviation for “revolutions per minute” is rpm as stipulated in CMOS 10.49. A document I’m editing contains a picture with the following label: “RPM Gauge.” I don’t like RPM being all capitals, but I’m not sure if rpm or Rpm is any better. What should I use?

Q. Hello, I’m looking for clarification for CMOS 10.33. Are you recommending 123 MAIN ST STE 456 for envelopes but 123 Main St., Ste. 456 for running text, etc.? (And if capitalizing the envelopes, would the entire address be capitalized?)

Q. I came across the following footnote in a scientific table: “[A] cohort born ≤ 2010, [B] cohort born ≥ 2011.” Is this an acceptable use of the ≤ and ≥ symbols?

Q. Can you use ’80s when referring to the 1880s? Thanks.