Summary: 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 [*bi*]lingual, [*bi*]ceps, and [*bi*]ped; the more technical Greek *di-* appears in such words as [*di*]phthong and [*di*]lemma. You can remember *bi-* easily by thinking of [*bi*]cycle, which has "two" wheels, and *di-* by remembering that the "dioxide" of carbon [*di*]oxide means that there are "two" oxygen atoms in the molecule CO2.
