Q. The February 2012 issue of National Geographic has a headline and subtitle that read, “What Dogs Tell Us: The ABC’s of DNA.” While I realize that National Geographic may have their own style guide, would Chicago style eliminate that apostrophe from ABC’s?

A. We would. But that apostrophe is conventional in newspaper and magazine publishing. You’ll probably see it everywhere, now that you’ve noticed it. In newspapers and magazines, where headlines often appear in all caps, the plurals of acronyms and initialisms without apostrophes (PCS, IVS, RBIS) would be difficult to interpret, since the final S would appear to be part of the acronym.