Bimonthly vs. Semimonthly

Does “bimonthly” mean twice per month or every two months? And what about “biweekly” or “biannual”? Do they mean alternating weeks or years, or do they mean twice in the same week or year?

The short answer is “yes.” These “bi–” words go both ways (and that’s a good way to remember it). This can lead to considerable confusion.

One way to reduce the level of confusion is to use “semimonthly” to mean every half month or twice a month, and to use “bimonthly” to mean every other month. Likewise, you can use “semiweekly” to mean twice a week, and “biweekly” or even “fortnightly” to mean every two weeks. Hence, a semiannual sale would happen twice a year, whereas the SPE Digital Energy Conference that alternates between the Amsterdam and Houston locations would be a “biennial” opportunity to the folks not allowed to fly overseas.

The trouble with this solution is that the “bi–” words historically go both ways, so you can’t rely on your audience understanding it your way. To remove confusion completely, you can just say “twice a week/month/year” or “every other week/month/year” and be crystal clear as to what you really mean.

Finally, when you are reading and run across one of these “bi–” words, keep in mind that there are two possible meanings, and if you cannot figure out which one is correct from the context, by all means ask for clarification, especially if you’re about to sign a contract.

Trivia Question of the Day:

Of all the vegetables, only two can live to produce on their own for several growing seasons. All other vegetables must be replanted annually. What are the only two perennial vegetables?

Asparagus and rhubarb.

2 Responses to “Bimonthly vs. Semimonthly”

  1. Jim Demers Says:

    “Semi”, for the majority of the populace, is equally ambiguous. It’s best to insert a parenthetical at the first occurrence of these terms; after that your audience will be on notice as to how you’re using it: “The committee will meet bimonthly (every other month) to review the project.”

    P.S.: kale and collard greens are also perennials.

  2. Daniel Lieberman Says:

    Since you brought up the term “perennial” here, it’s probably also worth mentioning that “biennial” is another great word, and a good option that always connotes every two years. This – at least – avoids confusion around which of two otherwise ambiguous words to use (“biannual” and “semiannual”).

    I’d love to follow your advice and always use bi- as every other and semi- to mean twice a… but the simple truth is that exactly half of my readers (or meeting invitees) will almost certainly misunderstand me (or show up on the wrong day).

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