The Couplet reference article from the English Wikipedia on 24-Apr-2004
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Couplet

Couplets are a rhyme scheme in which every couple of lines rhyme:

aa bb cc dd ee ff

and so on. It was used in Chaucer's Canterbury Tales in the 14th century. It became popular again in the eighteenth century with poets such as Dryden and Alexander Pope. Couplets with a meter of iambic pentameter are called heroic couplets.