Certain sets of words in the English language tend to confuse people. I have found the following to be among them:
BIANNUAL / BIENNIAL
Since annual means yearly and the prefix bi- means two, it is logical to assume that biannual would mean every two years. However, the preferred meaning for biannual is twice a year, the same meaning as semiannual, while biennial is the preferred word to mean every two years (just as bicentennial means every 200 years), especially when referring to flowering plants. That is why plants that flower every year are referred to as annuals while those that flower every two years are called biennials.
INCORRECT: Our neighborhood holds biennial block parties every year on New Year’s Day and the Fourth of July.
CORRECT: Our neighborhood holds biannual block parties every year on New Year’s Day and the Fourth of July.
INCORRECT: In the United States, biannual elections are held to elect members of the U.S. House of Representatives, who serve two-year terms.
CORRECT: In the United States, biennial elections are held to elect members of the U.S. House of Representatives, who serve two-year terms.
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