Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class I. Words Expressing Abstract Relations
Section V. Number
2. Determinate Number

89. Duality

   NOUN:DUALITY, dualism; duplicity; biplicity [rare], biformity; polarity.
  two, deuce, couple, couplet, both, twain, brace, pair, cheeks, twins, Castor and Pollux, gemini, Siamese twins; fellows; yoke, conjugation; dispermy, doublets, dyad, duad, twosome [rare], distich, span.
   VERB:[UNITE IN PAIRS] pair, couple, bracket, yoke; conduplicate; mate, span [U. S.]
   ADJECTIVE:TWO, twain; dual, dualistic; binary, binomial; twin, biparous; dyadic; conduplicate; duplex [See Duplication]; biduous, binate, binary, binal [rare], diphyletic, dispermic, paired, unijugate; tête-à-tête [F.].
  COUPLED &c. v.; conjugate.
  BOTH, both the one and the other.