Roget's Thesaurus: Entry 89 (Duality)

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#89. Duality. -- N. duality, dualism; duplicity; biplicity[obs], biformity[obs]; polarity. two, deuce, couple, duet, brace, pair, cheeks, twins, Castor and Pollux, gemini, Siamese twins; fellows; yoke, conjugation; dispermy[obs], doublets, dyad, span.

V. pair[unite in pairs], couple, bracket, yoke; conduplicate[obs]; mate, span [U.S.].

Adj. two, twin; dual, dualistic, double; binary, binomial; twin, biparous[obs]; dyadic[Math]; conduplicate[obs]; duplex &c. 90; biduous[obs], binate[obs], diphyletic[obs], dispermic[obs], unijugate[obs]; tete-a-tete.

coupled &c. v.; conjugate.

both, both the one and the other.