Prefix · two / double
Signals two or double: bicycle, bilingual, bilateral, bifocal, biannual.
Written by Bryan López, English teacher · Updated
Bi- comes from Latin "bis" (twice) and signals the presence of two elements, parts, or dimensions: bicycle = a vehicle with two wheels; bilingual = speaks two languages; bifocal = lenses with two different focal areas; bilateral = involving two sides or two parties. The core idea is always "two" in some sense.
Bi- applied to time periods can be ambiguous in English: bimonthly can mean "twice a month" or "every two months" depending on context. To avoid confusion, formal English prefers "twice a month" or "every two months." This ambiguity is genuine and acknowledged even by native speakers.
Attaches to nouns and adjectives with no spelling change. Identical in Spanish and English:
Pronunciation: /baɪ/, rhymes with "by" and "my." Identical in Spanish and English.
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
Bi- = "two": bicycle = two wheels, bilingual = two languages, bilateral = two sides, bifocal = two focal zones. Identical in Spanish and English. Watch out for biannual (twice a year) vs biennial (every two years): the difference is in the suffix, not the prefix.
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. In Spanish it usually maps to bi-.
The prefix bi- is pronounced /baɪ/. For example, "bicycle".
"cycle" becomes "bicycle". It is a typical example of the bi- prefix.
un-, re-, pre-, dis-, over-, in-... every beginning you need to unlock thousands of English words at once.