[1] : The Collaborative International Dictionary of English v.0.48

duo \du"o\, n. it. duo, fr. l. duo two. see duet. mus.
   a composition for two performers; a duet. duodecahedral
see also:
duet 

[2] : WordNet (r) 2.0

duo
     n 1: two items of the same kind syn: couple, pair, twosome,
           twain, brace, span, yoke, couplet, distich,
           duet, dyad, duad
     2: two performers or singers who perform together syn: duet,
         duette
     3: a pair who associate with one another; "the engaged couple";
        "an inseparable twosome" syn: couple, twosome, duet
     4: a musical composition for two performers syn: duet, duette
     also: dui pl
see also:
couple pair twosome twain brace span 
yoke couplet distich duet dyad 
duad duette dui 

[3] : Moby Thesaurus II by Grady Ward, 1.0

38 moby thesaurus words for "duo":
   both, brace, couple, couplet, distich, double harness, doublet,
   duad, duet, duettino, dyad, ensemble music, ensemble singing,
   match, mates, octet, pair, part music, part singing, part song,
   polyphonic music, quartet, quintet, septet, septuor, sestet,
   set of two, sextet, span, team, terzet, terzetto, the two, trio,
   twain, two, twosome, yoke

Duo >> D Words
Duo, definition of term: Duo
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