If you receive a paycheck on the fifteenth and thirtieth of each month, are you paid bimonthly or semimonthly?

If a newspaper is published every two weeks, it is a biweekly or a semimonthly publication?

The answer is not simple.

If we check Webster’s for definitions of each prefix, we find that although semi always means “half,” the prefix bi can mean either “every two” or “twice.” Thus, bimonthly can mean either “every two months” or “twice a month.”

Webster’s warns us that when using the prefix bi, we should give the reader clues about which meaning we intend. And when you are the reader, you should make sure you know which meaning the writer intends.

Before you decide to pay $19.95 for a bimonthly magazine, read the fine print to find out if you will receive six issues a year or twenty-four.

When we mean twice (as in “twice a week” or “twice a month”), we can avoid ambiguity by using the prefix semi (as in semiweekly for “twice a week” and semimonthly for “twice a month”).

Three final notes:

  1. We have a special word—biennial—that means “occurring every two years.” The word biannual has only one meaning: “occurring twice a year.” Biannual is thus interchangeable with semiannual, although biannual is less preferable if we wish to avoid any chance of misreading by folks who are not thoroughly familiar with these definitions.
  2. Never use a hyphen with either of these prefixes unless the root word begins with the letter i, as in semi-independent, semi-invalid, and semi-infinite.
  3. English speakers outside of the United States frequently employ the very useful word fortnight, which unambiguously describes a period of two weeks.

Which prefix—bi or semi—would be the better (that is, the less ambiguous) choice for a word to describe each situation below?

  1. occurring twice a week
  2. occurring every two months
  3. occurring every two years
  4. occurring twice a year
  5. occurring twice a month
  6. occurring every two weeks
  1. semiweekly (Biweekly is acceptable but potentially ambiguous.)
  2. bimonthly (Be careful to provide context clues so the reader does not think you mean “twice a month.”)
  3. biennial
  4. semiannual (Biannual is also correct but potentially ambiguous to readers unfamiliar with the difference between biennial and biannual.)
  5. semimonthly (Bimonthly is acceptable but potentially ambiguous.)
  6. biweekly (Something that occurs every two weeks happens more often than something that happens twice a month since a year comprises 52 weeks but only 12 months.)

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