Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
duo (n.), duo- (prefix)
Duo, the noun, has two plurals: the regular duos and the almost never-used dui. A duo is two, a pair, a couple: They made a duo around town for the better part of the year. The word has an Informal or Conversational air in some uses, probably because it looks as though it were the result of clipping or another process of abbreviation (it’s not), but it is Formal in music when it refers to two performers or to a piece of music for two performers or two instruments (a duo for violin and viola). The prefix duo- occurs in words such as duodecimal (two plus ten), duologue (conversation for two; we’ve usually preferred dialogue instead), and duopoly (a rare term for a ruling political combination of two powers).
