Context: I am looking for a term to indicate a time period of 3 weeks/21 days

For instance, a "fortnightly" event would occur every 2 weeks/14 days.

My Usage:

The "Read for the Visually Challenged" is a {fill in word for 3 weeks} event as part of CSR initiatives organised by our comapny.

When I searched online, I stumbled upon Triweekly, an adjective with two entirely different meanings

  1. occurring or appearing three times a week
  2. occurring or appearing every three weeks


Ideally, I mean #2, but will this create a confusion? Why does it mean 2 different time periods? Is there another word to indicate a period of exactly 3 weeks?

P.S: I am not looking for "monthly" since this event is conducted every 3 weeks and it could be possible that it occurs in the first and fourth week of the same month.

  • Related: An event that recurs four times a year? – Tim Lymington Dec 29 '15 at 13:31
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    If it didn't have both meanings then it wouldn't be consistent with biweekly and bimonthly. – Hot Licks Dec 29 '15 at 13:55
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    xkcd.com/1602 obligatory xkcd – Pål GD Dec 29 '15 at 14:12
  • I'd avoid any terms that divide a week, since there's no way to do it evenly - 7 is not a multiple of 2 or 3 (or anything else since it's prime). True, 365 is also not a multiple of 2 or 3 either, but you can get a lot closer to a half- or third-year than a half- or third-week. – Darrel Hoffman Dec 29 '15 at 15:07
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    Sesquifortnightly? – Octopus Dec 29 '15 at 22:14

Confusingly (according to dictionary definitions) the same is also true of biweekly; bimonthly; and biyearly. All of them can mean once every two... or twice per...

In the case of biannual the OED gives its adjectival meaning as once every two years, but when used as a noun as meaning the same as biennial, i.e. every other year. Semi-annual can be often used to express "twice per year".

In view of the dichotomous meanings of biweekly, triweekly etc. all I can suggest is that where there is likely to be confusion that you avoid their use, in favour of once every three weeks etc. The OED recommends the use of the terms semi-weekly, semi-monthly, semi-annually to avoid any confusion when twice per... is intended.

  • Thanks for the answer. Any thoughts on why or how two different meanings for the same word came about in the first place? – BiscuitBoy Dec 29 '15 at 9:56
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    @BiscuitBoy The OED doesn't say. I suppose it was just that people started using them both ways until each method developed its own purchase. But the OED does comment: bi-weekly adj. (b) Occurring or appearing twice in a ——; as in (The ambiguous usage is confusing, and might be avoided by the use of semi-; e.g. semi-monthly, semi-weekly; cf. half-yearly adj.) – WS2 Dec 29 '15 at 10:35
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    -1 Uh, I thought biannual means twice a year. In fact I never thought it means once every two years. – user541686 Dec 30 '15 at 0:57
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    This is just wrong about years. A biannual event occurs twice per annum; a biennial one occurs twice each two years. I have never heard or used 'biyearly', but it appears to be in usage ambiguously - so why bother when biannual and biennial are unambiguous I don't know! As is so often the case: add more Latin for less ambiguity! – OJFord Dec 30 '15 at 1:01
  • @Mehrdad I would agree that there is confusion over this. The OED gives the adjective biannual as meaning twice per year, but the noun as having the same meaning as biennial. I will edit my answer accordingly. – WS2 Dec 30 '15 at 9:20

"Every three weeks" is the most unambiguous option.

The problem appears to be in the semantic nature of the prefixes which carry the double meanings:


  • word-forming element meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek treis, trias "three".


  • word-forming element meaning "two, twice, double, doubly, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, Old English twi- "twice, double"), from PIE root *dwo- "two." Nativized from 16c. Occasionally bin- before vowels; this form originated in French, not Latin, and might be partly based on or influenced by Latin bini "twofold".


Usage note:

  • All words except biennial referring to periods of time and prefixed by bi- are potentially ambiguous. Since bi- can be taken to mean either “twice each” or “every two,” a word like biweekly can be understood as “twice each week” or “every two weeks.” To avoid confusion, it is better to use the prefix semi- to mean “twice each” ( semiannual; semimonthly; semiweekly) or the phrase twice a or twice each (twice a month; twice a week; twice each year), and for the other sense to use the phrase every two (every two months; every two weeks; every two years).
  • Actual usage suggests that the term "triweekly" is not commonly used, probably because of its ambiguous nature: see Ngram.
  • And as two is to semi_, three is to ?. – WS2 Dec 29 '15 at 13:18
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    @WS2 - Obviously, road train. – Hot Licks Dec 29 '15 at 13:57
  • I'm sure triweekly is uncommon simply because it's not needed very often. I, for one, can't recall the last time I thought about an event that happened three times a week, or once every three weeks, and I can't think of any now. – talrnu Dec 29 '15 at 19:20
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    @talrnu - actually three times a week is a fairly common expression according to Ngram – user66974 Dec 29 '15 at 19:31
  • Fair enough - perhaps you're right, then. – talrnu Dec 29 '15 at 19:35

The only clear way to say it is "every three weeks" (or something similar). If you use words like triweekly, you will just confuse everyone. Even if their interpretation of the word is correct, they will be unable to rely on your interpretation also being correct and matching theirs.

  • The correct answer. – Fattie Dec 29 '15 at 23:26

Why there are two different meanings for “triweekly”?

It's almost as though the language evolved rather than being properly designed.

Is there another word to indicate a period of exactly 3 weeks?

Yes, "three-weekly".

And for the other meaning (three times a week): "thrice-weekly".


1x3, three a week

3x1 = 3 weeks or once every 3 weeks.

It's short, it defies language barriers, and it works.

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