I got a notice from my vet that my dog was due for a biannual procedure. It’s actually done every six months, so I finally suggested maybe they should call it a semiannual procedure. No, the office manager said. They looked it up years ago, and biannual means twice a year.
Now let’s see. Semi- means half. Bi- means two. Logically, then, semiannual ought to refer to half a year and biannual ought to mean two years. But we’re dealing with the English language here.
I looked it up. The office manager is correct, at least according to most web sites I found. Biannual means semiannual. Semiannual means every half year, but biannual doesn’t mean every two years. It means two times a year. But semiannual doesn’t mean half a time a year.
What do you do if you want to describe something that happens every two years? You have to use biennial. That word makes some sense. After all, millennial refers to a thousand years and centennial refers to a hundred years. So of course biennial refers to two years.
So we have two different prefixes for “annual” that mean the same thing and one prefix that means different things on two related stems. Clear?
Other units of time
Those same two prefixes are used with other units of time. Offhand, I don’t recall seeing either bidaily or semidaily, although Microsoft Word’s spell checker flags semidaly but not bidaily. Biweekly means every two weeks. At the moment, I don’t remember seeing semiweekly either, but Word isn’t objecting to it. Someone must use it. The Associated Press style guide explains how.
Someone who gets paid biweekly receives 26 paychecks a year, probably on alternate Fridays. Someone who get paid bimonthly gets 24 paychecks a year, probably on the 15th and last day of every month. That is, twice a month. But if someone subscribes to a bimonthly magazine, it comes six times a year, that is, every two months. Somehow we figure it out.
A worse confusion
At least the confusion of “biannual” isn’t as bad as “inflammable.” That word properly refers to something that can be inflamed. Inflammable was the standard way to refer to something that can be set on fire for hundreds of years. Something that cannot be set on fire was always called nonflammable.
But most people over the past century or so take the prefix in- as meaning not. In other words, inflammable looks like it means that it won’t burn. Misunderstanding that word can be fatal. Today, the preferred term in flammable. Somewhere in the back of my mind, I recalled that it is illegal to label any product inflammable, but I can’t verify it now.
But back to biannual vs semiannual, it would make sense to prefer semiannual. It would make sense to refer to getting paid on the 15th and last day of every month as semimonthly. But, as I say, we’re talking about the English language. English usage doesn’t have to make sense. Usually, it’s easy enough to figure out the meaning of biannual or bimonthly in context.