In a Word: Hemi, Semi, Demi, Bi, and Di

Adding math to language can lead to confusion.


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Senior managing editor and logophile Andy Hollandbeck reveals the sometimes surprising roots of common English words and phrases. Remember: Etymology tells us where a word comes from, but not what it means today. 

Math and language don’t always play well together. Take, for instance, how confusing it has become to be clear about words that involve either dividing or multiplying by 2. 


Greek has the prefix hemi- meaning “half.” Hemisphere — half a sphere — is probably the most common use of this prefix. There are dozens of other examples, but they’re primarily technical terms used in science and medicine, not exactly everyday words.  

Now, Latin has a different prefix for “half”: semi-. The similarity between the two isn’t mere coincidence; historical linguists believe that both can be traced back to the same root in the theoretical precursor language Proto-Indo-European. You’ll find semi- in a lot of common words, such as semicircle and semifinal 

Because semi- has seen more use among us common folk, it has a more diluted sense as well: As often as not, semi- can indicate “partly” (as in semi-automatic and semidarkness) or “almost, but not quite” (as in semipermanent and semitransparent), and not exactly half of something. (Quasi-, from a Latin word meaning “as if,” could be more etymologically accurate in those cases, but it became popular as an English prefix only over the last two centuries, well after many semi- words were already common.) 

And then there’s demi-. It, too, means “half,” but despite its similarity to hemi- and semi- in both appearance and meaning, it isn’t etymologically related. Demi- stems from dimidius “half,” which combines dis- “apart” and medius “middle.” Because if you take something apart at the middle, you’ve got two halves, right? Dimidius became dimedius in Late Latin, which became demi in French, which is what English picked up as a prefix meaning “half,” as if we didn’t already have enough of those. 

The Greek and Roman gods weren’t known for their fidelity, so you’ll find a lot of characters in those mythologies who are demigods, literally “half-gods” because they are the offspring of a god and a mortal. And those tiny coffee cups that espresso is served in are called demitasse; tasse is French for “cup,” so they’re literally “half-cups.” 


In Latin, bis means “twice, double.” This form shows up as a prefix in some English words (bissextus, for example), but the more common form is bi-, as in bicycle, bifocals, and bifurcated (like a snake’s tongue).  

But wait! There’s a Greek version, too, and like semi- and hemi-, it looks quite similar. The Greek word for “twice” is dis, which, as a prefix in English, is shortened to di-. It’s the first part of a dilemma, in which one is faced with two equally unpleasant choices. And carbon dioxide is so called because it’s made up of one carbon atom and two oxygen atoms.  

(And that is as far as I will go into chemistry, where the prefixes bi- and di- have very narrow technical meanings that give me a headache to even think about.) 


The ever-present confusion with these prefixes comes when we use them to indicate time. I’m sure you’ve been there before: Is that biweekly meeting twice a week or every other week? And what’s up with biennial, biannual, and semiannual? 

Maybe this can help: The prefixes bi- and di- are two letters long, and they mean “two.” The prefixes semi-, hemi-, and demi- all contain four letters, just like the word half. 

So biweekly means every two weeks, and semiweekly means every half-week — or twice a week. Of course, because the language is so muddled through misuse and misunderstanding, you’ll probably need to be more explicit if you want to make sure people show up when they’re supposed to: There’s nothing unclear about twice-weekly meetings, and for the two-week gap, you could use semimonthly (though that would indicate 24 meetings a year instead of 26) or the more interesting fortnightly (from a shortening of “fourteen nights”). 

And about mathing years: Biennial (literally “two years”) indicates, as you would expect, something that lasts for two years or happens every two years. Semiannual indicates a half-year, so it’s something that happens approximately every six months.  

Biannual is the one odd; etymologically, it ought to mean “every two years” — same as biennial — but it has a long history of being used to mean “twice a year.” If you use biannual, someone will be unsure of what you mean. You don’t have to use it, though. And in fact, if everyone stopped using biannual, if it became obsolete, the English language would be all the clearer for it.  

Speaking of fortnightly, to go back a bit: In a Word is henceforth moving to a fortnightly publication schedule. That is, biweekly. If you thought combining math and language was interesting, come back in two weeks and I’ll throw music into the mix, too. 

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  1. This is one of your best columns yet. Honestly, I’ve really never heard ‘hemi’ used other than the famous Chrysler Corp. engine, except hemisphere, of course. Demi, semi too. Oh those Latins and Greeks. Biennial, biannual, semiannual, fortnightly are all great words. There just aren’t enough occasions to use them very often.


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