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Twice, two, double.

Greek dis, di‑, two or twice.

Some older examples derive from Greek words already containing the prefix, such as diphthong, a sound formed by the combination of two vowels in a single syllable (Greek diphthongos, from phthongos, voice or sound), or dilemma, a situation in which a difficult choice has to be made between two or more equally undesirable courses of action (Greek dilēmma, from lēmma, premise).

On these models, a number of English forms have been created, mostly technical terms. Examples include dipole, a pair of equal and oppositely charged or magnetized poles separated by a distance; Diptera (Greek pteron, wing), a large order of insects that comprises the two-winged or true flies; dimer (formed on the pattern of polymer) a molecule or molecular complex consisting of two identical molecules linked together; dioecious (Greek ‑oikos, house), describing a plant or invertebrate animal that has the male and female reproductive organs in separate individuals.

In chemistry, di‑ is used to indicate the presence of two atoms or groups of a specified kind, as in dioxide, dichromate, disulphide and many others. It is also used to make compound affixes indicating that a radical appears twice in a molecule, as in dichloro‑, ‑diene, dihydro‑, dimethyl‑, ‑dione, and diphenyl‑. All of these except the last have separate entries under the radical name without the preceding di‑.

For dichotomy, see dicho‑. Some words beginning in di‑ contain instead the Latin prefix meaning ‘apart’, derived from dis‑: digest, dilapidated, divert. Other examples are in the next entry.

See also bi‑.

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