1580s, "song for two voices, duet," via either Italian or French from Latin duo "two" (from PIE root *dwo- "two"). Meaning "two people" (especially as an entertainment team) attested by 1887.

1724, "composition for three voices," from French trio (c. 1600), from Italian trio, from tri- "three" (see three); patterned on duo. Meaning "group of three persons" is from 1789.

Proto-Indo-European root meaning "two."

It forms all or part of: anadiplosis; balance; barouche; between; betwixt; bezel; bi-; binary; bis-; biscuit; combination; combine; deuce; deuterium; Deuteronomy; di- (1) "two, double, twice;" dia-; dichotomy; digraph; dimity; diode; diphthong; diploid; diploma; diplomacy; diplomat; diplomatic; diplodocus; double; doublet; doubloon; doubt; dozen; dual; dubious; duet; duo; duodecimal; duplex; duplicate; duplicity; dyad; epididymis; hendiadys; pinochle; praseodymium; redoubtable; twain; twelfth; twelve; twenty; twi-; twice; twig; twilight; twill; twin; twine; twist; 'twixt; two; twofold; zwieback.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."

"twofold nature, state of being two or divided in two," late 14c., from Late Latin dualitas, from Latin dualis "that contains two; the dual number, duality," from duo "two" (from PIE root *dwo- "two").

late 15c., "to repeat, produce a second (like the first);" 1620s, "to double," from Latin duplicatus, past participle of duplicare "to double," from duo "two" (from PIE root *dwo- "two") + plicare "to fold" (from PIE root *plek- "to plait"). Related: Duplicated; duplicating.

1919, in electricity, "tube with two electrodes," from Greek di- "twice" (from dis "twice," related to duo, from PIE root *dwo- "two") + hodos "a way, path, track, road," a word of uncertain origin (see Exodus).

early 15c., "having two parts, double," from Latin duplicatus, past participle of duplicare "to double," from duo "two" (from PIE root *dwo- "two") + plicare "to fold" (from PIE root *plek- "to plait"). Meaning "exactly corresponding, that is an exact copy of" is from 1812.

before vowels, di-, word-forming element meaning "through, in different directions, between," also often merely intensive, "thoroughly, entirely," from Greek dia "through; throughout," probably cognate with bi- and related to duo "two" (from PIE root *dwo- "two") with a base sense of "twice."

c. 1600, in grammar, "the form or number relating to two," from Latin dualis "that contains two; the dual number, duality," from duo "two" (from PIE root *dwo- "two"). General sense of "relating to two, expressing two, composed or consisting of two parts" is from 1650s. Related: Dually.

"the number two, two units treated as one," 1670s, from Latin dyad-, stem of dyas, from Greek dyas "the number two, a group of two," from duo "two" (from PIE root *dwo- "two"). Specific sense in chemistry ("a bivalent element") is by 1865; also used in biology, poetics, mathematics. Related: Dyadic.