The English prefixes bi-, derived from Latin, and its Greek variant di- both mean “two.” The Latin prefix is far more prevalent in common words, such as bilingual, biceps, and biped; the more technical Greek di- appears in such words as diphthong and dilemma. You can remember bi- easily by thinking of bicycle, which has “two” wheels, and di- by remembering that the “dioxide” of carbon dioxide means that there are “two” oxygen atoms in the molecule CO2.
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