One of my Twitter followers (I can’t bring myself to refer to them as tweeps in public, sorry) emailed me with a question about the proper use of the Latin abbreviation “ca.” It seems she’d worked with an author who claimed he had never encountered it before despite being “someone with a graduate degree, [who has] written thousands of papers, read 100s of books.”

Forgive my incredulity, but all right; he says he’s never seen it.

I’ve seen it plenty and I don’t have a graduate degree. I see it in periodicals like National Geographic and Smithsonian. I see it in resources like encyclopedias. I see it all over.

But in any case, here’s the deal.

“Circa” is Latin, of course, and it’s abbreviated “ca.” Either way, the meaning is “about” or “approximately.” It can sometimes be abbreviated with simply a lowercase c, but that can be confused with “copyright” so the two-letter version is preferred. Whether it’s written out or abbreviated, it’s not italicized.

So, when is this “ca.” used? With dates, and sometimes with measurements. Most often, though, you’ll see it with dates, particularly (in my experience) in the areas of geneaology and archaeology. If a particular dig yields artifacts that can’t be precisely dated for whatever reason, you might see reports of “a bronze bowl, ca. 2500-1600 B.C.” That’s how I first encountered this usage. I think I was in sixth grade, so that’d have been 1968. (This is why I have trouble believing the fellow who made the claims at the start of this post. It doesn’t make sense to me. Perhaps I’m odd.) You’ll see it on websites as well, as shown in that hyperlinked text to the Metropolitan Museum of Art in New York City.

Use it if you’re uncertain about a date, but be careful.  If you write “ca. 1889–1975,” readers may take that to mean you’re unsure about both dates, not just the first one. It’s probably better to write “born ca. 1889, died 1975” to be clear. (Unless, of course, you really do mean you’re not sure of either one.)

In my experience it’s used much less often with measurements, but here’s a link to an example from Google Books (The Measure of Form: A New Approach for the Study of Indian Sculpture, Mosteller).

Notice that in either case (with dates or with measures), “circa” or “ca.” appears before the numerals, never after.

That’s about all I can come up with for discussion of this abbreviation. Leave me a comment if you like, if you think I’ve forgotten something or misspoken or what have you. I’m always up for clarification and education!