The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
dwo-
DEFINITION:
Two. Derivatives include twilight, biscuit, between, combine, diploma, and doubt. I. Variant form *duwo.1a.two, from Old English tw, two (nominative feminine and neuter); b.twain; twayblade, from Old English twgen, two (nominative and accusative masculine). Both a and b from Germanic *twa, two. 2.twelfth, twelve, from Old English twelf, twelve, and twelfta, twelfth, from Germanic compound *twa-lif-, two left (over from ten), twelve (*-lif-, left; see leikw-). II. Adverbial form *dwis and combining form *dwi-.1a.twibill, twilight, from Old English twi-, two; b.zwieback, zwitterion, from Old High German zwi-, twice. Both a and b from Germanic *twi-.2. bi-1, bis; balance, barouche, bezel, biscuit, bistort, from Latin bis (combining form bi-), twice. 3. di-1, from Greek dis (combining form di-), twice. 4.twist, from Old English -twist, divided object, fork, rope, from Germanic *twis.5.twice, from Old English twige,twiga, twice, from Germanic *twiyes.6.twenty, from Old English twntig, twenty, from Germanic compound *twgentig, twice ten (*-tig, ten; see dek). 7.twine, from Old English twn, double thread, from Germanic *twhna, double thread, twisted thread. 8.between, betwixt, twixt, from Old English betwonum and betweox,betwix, between, from Germanic compounds *bi-twhna and *bi-twisk, at the middle point of two (bi, at, by; see ambhi). 9.twill, from Old English twilic, woven of double thread, from Germanic compound *twilic-, two-threaded fabric. 10. Suffixed form *dwis-no-.a.twin, from Old English twinn,getwinn, two by two, twin, from Germanic *twisnaz, double; b. bi-1, binal, binary; combine, pinochle, from Latin bn, two by two, two each. 11. Suffixed form *dwi-ko-.twig1, from Old English twigge, a branch, from Germanic *twig(g)a, a fork. 12. Compound *dwi-plo-, twofold (*-plo-, -fold; see pel-2). diplo-, diploe, diploid, diploma; anadiplosis, diplodocus, from Greek diploos,diplous, twofold. 13. Suffixed reduplicated form *dwi-du-mo-.didymium, didymous; epididymis, from Greek didumos, double, the testicles. 14. Suffixed form *dwi-gha.dichasium, dicho-, from Greek dikha, in two. III. Inflected form *duw.1.deuce1, dozen, dual, duet, duo, duo-; duodecimal, duumvir, from Latin duo, two. 2.duad, dyad; dodecagon, hendiadys, from Greek duo,du, two. IV. Variant form *du-.1. Compound *du-plo-, twofold (*-plo-, -fold; see pel-2). double, doublet, doubloon, duple, from Latin duplus, double. 2. Compound *du-plek-, twofold (*-plek-, -fold; see plek-). duplex, duplicate, duplicity; conduplicate, from Latin duplex, double. 3. Suffixed form *du-bhw-io-.doubt, dubious; redoubtable, from Latin dubius, doubtful (< hesitating between two alternatives), and dubitre, to be in doubt. (Pokorny d(u) 228.)