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Affixes: the building blocks of English
Affixes: the building blocks of English

di-

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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