And they do, quite often.

My current project uses APA (also called, colloquially, “science”) style. Now I’m a CMoS gal, and I know AP pretty well, but even when I had to write reference papers in APA style for my most recent degree work, I didn’t run up against this particular guideline that’s driving me bats.

More bats than usual, that is.

APA style says that “U.S.” is for adjectival use (“the U.S. navy”), and “US” is for noun use (“they traveled across the US in a Winnebago”).

Why?

What possible reason is there for this differentiation? What purpose does it serve? Do readers need some kind of visual signal that the abbreviation’s being used adjectivally? I can’t figure out the purpose here, at all.

Except to enrage copy editors.

And give proofreaders something to look for.

CMoS prefers “US” across the board, but allows for periods if the author and editor wish to use them. Across the board. Use them, don’t use them, but be consistent.

EDIT: That’s misrepresented. CMoS prefers “US” as the abbreviation when other US postal codes are being used (IL, WI, MN, etc.), but would rather we use the full name written out (United States) as a noun. That makes sense to me, honestly. Furthermore, if the old-fashioned traditional abbreviations are used (Ill., Wis., Minn., etc.), then CMoS says we should use “U.S.” — which, again, makes sense to me. That’s consistency. Logical consistency doesn’t bother me. I rather like it. (Thank you to Twitter user Ashley Bischoff, @FriendlyAshley, for bringing this to my attention.) It goes back to my generalization: Use them, don’t use them, be consistent. (And seriously: Even if you decide to go against the CMoS guidelines, as long as you’re consistent it’s unlikely readers will notice. They notice inconsistency.)

AP says to omit periods in headlines, but use them in text (body copy). That makes sense, given the restrictions on space in headlines in printed material (less so in electronic, but still, okay). There’s a logic to that.

MLA says use periods across the board. That also makes sense, and is consistent.

So what’s up with this, APA? Anyone out there care to explain? I mean, really explain, not just offer up conjecture. Because I don’t get this one, at all.