Normally, the prefix "bi-" is pretty straightforward; it means "two." However, through an unfortunate quirk of English's evolution, this prefix has taken on two meanings in relation to measures of frequency.* It can mean "once every two ____" or "twice per ____" (where "____" is a unit of time). Similarly, the prefix "tri-" can mean either "once every three ____" or "three times per ____" in some cases.
As a result, several "bi-" and "tri-" words are inherently ambiguous. The simplest solution is to avoid these terms at all times to prevent confusion:
"biannual" and "triannual"
"biweekly" and "triweekly"
"bimonthly" and "trimonthly"
Fortunately, other uses of "bi-" and "tri-" are clear, so you don't have to worry about ambiguity unless you are referring to frequency.
For well over a century, the prefixes "bi-" and "tri-" have been associated with dual meanings in regard to frequencies. Unfortunately, checking a dictionary is of no help, as most of these terms have multiple, contradictory definitions. See the entries for "triweekly" and "bimonthly" for examples. As a result, phrases such as the following one can have contradictory meanings.
the biweekly sale (can refer to a sale that occurs twice per week or every two weeks)
Some authorities say that frequency words with these prefixes can be acceptable if used with proper context clues, but this solution is not a strong one. After all, if you are providing sufficient context clues in the sentence for the reader to understand how often the event occurs without reference to the frequency word, then the frequency word is not necessary at all and should simply be omitted. See this example:
I missed the biweekly sale, so now I have to wait two weeks to try to buy the sweater again.
In this case, the context clue "two weeks" does clarify the meaning of "biweekly," but the sentence would have the same meaning (and be more concise) if "biweekly" was left out entirely:
I missed the sale, so now I have to wait two weeks to try to buy the sweater again.
Thus, the simplest and clearest solution is to just avoid "bi-" and "tri-" frequency words at all times (with the exception of words using the "enn" and "mes" roots; see below). Instead, use terms that are clear on their own—and there are many such alternatives, as I show below. For now, consider how you might fix these example sentences to make them concise and unambiguous:
The meetings occur trimonthly—on the first, third, and fourth Tuesday of each month. To reduce waste, the company signed up for biweekly recycling service. For the last decade, I have traveled to Asia biannually. My secret to shiny hair is an oil-based serum that I apply triweekly. Shayla's Salon offers a discount for clients who sign up for bimonthly manicures. The triannual competition features participants from 20 countries.
The frequency words "biweekly" and "bimonthly" are adjectives and adverbs. On the other hand, "biannual" is only an adjective; "biannually" is its adverbial form. I bring this up because the best replacements for these terms may depend on the part of speech.
If you are referring to an event that occurs once per two weeks/months/years, then the prefix "semi-" (meaning "half") can usually be used in place of "bi-": The forms "semimonthly" (every two months) and "semiweekly" (every two years) are both adjectives and adverbs; use "semiannual" as an adjective or "semiannually" as an adverb.
There is no equivalent to "semi-" for an event that occurs once per three weeks/months/years. In some cases, there are single-word replacements (such as "triennial" for "triannual"; see below), but in other cases, you will need to write out a clear adverbial phrase such as "every three months."
Interestingly, although terms such as "biannual" and "trimonthly" are ambiguous, the related words "biennial" and "trimester" each only have one meaning ("every two years" and "three months," respectively). This is because the roots "enn" (year) and "mes" (month) are only ever used in constructions that mean "every XX years" and "every XX months," so there can be no confusion. As a result, words using the roots "enn" or "mes" after the prefixes "bi-" or "tri-" are fine to use and can make good replacements for their ambiguous cousins—provided that their meaning matches your intent.
For events that occur twice or three times in a given period, you can use hyphenated phrases as adjectives (e.g., "twice-weekly" or "thrice-yearly"); as adverbs, you can use those same phrases with spaces instead of hyphens (e.g., "twice weekly" or "thrice yearly"), or you can use phrases with "per" (e.g., "twice per week" or "three times per year").
Now, let's return to the incorrect example sentences and try to fix them. The first example sentence provides the exact times of the meetings (underlined below), so "trimonthly" is redundant and should be removed:
The meetings occur on the first, third, and fourth Tuesday of each month.
The other five sentences are ambiguous, and each can be written two ways, depending on what is meant.
Here are the two versions of each of the other sentences, in turn:
To reduce waste, the company signed up for twice-weekly recycling service. To reduce waste, the company signed up for semiweekly recycling service.
For the last decade, I have traveled to Asia twice per year. For the last decade, I have traveled to Asia semiannually. (or biennially)
My secret to shiny hair is an oil-based serum that I apply three times per week. My secret to shiny hair is an oil-based serum that I apply every three weeks.
Shayla's Salon offers a discount for clients who sign up for twice-monthly manicures. Shayla's Salon offers a discount for clients who sign up for semimonthly manicures.
The competition features participants from 20 countries and occurs three times per year. The competition features participants from 20 countries and occurs every three years.
If you can't think of an appropriate word to replace a "bi-" or "tri-" frequency word, it is always fine to recast the sentence using a phrase, as I did for both versions of the last example sentence. Concision is important, of course, but it is less important than being clear and unambiguous.
Thank you for reading this post; I hoped it helped you understand the issue with words such as "bimonthly" and gave you the tools to avoid such words in your writing. As always, if you have any questions, you can contact me in the comments or by email (firstname.lastname@example.org). If you prefer to have me improve your writing by removing ambiguity—and doing so much more—simply request a quote from ElevationEditing.com. See you next time!