There seems to be a fair amount of confusion surrounding the meaning of the prefix bi when used with units of time measurement. Biweekly, according to dictionary.com, can mean either "occurring twice weekly," or "occurring once every two weeks." Falling within the same realm of confusion are bimonthly and biannually.

As of late, it seems that we've begun replacing the bi prefix with semi when the intended meaning is "occurring twice within," while continuing to use the former when the intended meaning is "occurring once every other." Though this is a step in the right direction, there still seems to be confusion around proper usage.

Whose genius idea was it to start using the bi prefix to mean two very different things? If my employer told me my pay periods were bimonthly, should I assume they meant semimonthly or once every other month?

closed as not a real question by Hellion, kiamlaluno, Robusto, Kosmonaut Mar 10 '11 at 2:35

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    This question sounds an awful lot like peeving disguised as a question. What are you actually asking here? The identity of the single person who coined a novel usage? – nohat Mar 10 '11 at 0:32
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    I interpret the core of this question as asking "am I right to think that bi- words are currently ambiguous? do most people take them as twice an x, or as once every two x?". This could give helpful answers to potential visitors from the internets. – Cerberus_Reinstate_Monica Mar 10 '11 at 0:53
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    @Cerebrus Yup, I agree. We should interpret this post in the most charitable light, and I think there is a germ of a good question here... – Uticensis Mar 10 '11 at 1:36
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    I'm most disappointed that the title wasn't "biweekly, bimonthly, bi-curious." :-) – Hellion Mar 10 '11 at 2:01
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    As you showed, the bi- prefix is ambiguous in this context, so there is no answer for your second question. You can't assume either. (And your first question, of course, is actually just a complaint :) – Kosmonaut Mar 10 '11 at 2:36

The obvious distinction is between biannual and biennial which although originally cognate from Latin and Italian now seem to have distinct common meanings: twice a year and every two years respectively, even though Wiktionary discourages biannual.

So for weeks and months all you need to do is wait for two forms to arise and then wait for people to decide which is which. Semi- for "half a ...", i.e. "twice a ...", looks a sensible approach.


Looking at the prefixes, bi means two, and semi means half. But the question here, is does that refer to the number of occurrences in the given period, or the number of periods per occurrence? I think that's where the confusion came in, and apparently the coiners of the words were not consistent.

  • Unfortunately, I'm not sure how we can help to reduce the confusion. – Craig Buchek Mar 10 '11 at 1:37
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    If used as the Romans would, bi-x would mean "once every two x". – Cerberus_Reinstate_Monica Mar 10 '11 at 1:51

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