Prefix · two / double
Signals two or double: bicycle, bilingual, bilateral, bifocal, biannual.
Written by Bryan López, English teacher · Updated
Bi- comes from the Latin bis, which means "twice". It shows that there are two elements, two parts or two dimensions. The central idea is always "two". If a word starts with bi-, look for the "two" and you will almost always guess the meaning.
When bi- is used with periods of time, it can confuse even native speakers. For example, a bimonthly magazine can come out twice a month or once every two months, depending on who is speaking. The same happens with biweekly. To avoid mistakes, formal English prefers to say "twice a month" or "every two months".
Attaches to nouns and adjectives with no spelling change:
bi-/baɪ/
Tap the button to hear how the prefix sounds. Each word in the table has its own audio.
"Biannual" = twice per year (bi + annual). "Biennial" = every two years (bi + ennial). The suffix changes the meaning: annual = per year, ennial = two-year cycle.
This ambiguity is recognised even by native English speakers. In formal contexts, "twice a month" or "every two months" are preferred to avoid misunderstanding.
A trick to remember it
Think of a bicycle, which has two wheels: bi- = two. bilingual, biannual.
Exercise 1 · Form the word
Exercise 2 · Pick the right one
"A ___ has two wheels and is powered entirely by the rider's own pedalling effort."
Exercise 3 · Pick the right one
What is the difference between "biannual" and "biennial"?
The prefix bi- signals two or double: bicycle, bilingual, bilateral, bifocal, biannual.
The prefix bi- is pronounced /baɪ/. For example, "bicycle".
"cycle" becomes "bicycle". It is a typical example of the bi- prefix.
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