dwóh₁

PIE word for two.‌​‍​‍​‌​‍​‌​‍​‌​‍​‍​‍​‍​‌​‌​‌​‌​‍​‍​‌​‌​‍​‌​‌​‌​‌​‍​‍​‌​‌​‌​‌​‌ Gives Latin duo, English "two", "twin", Greek dúo, Sanskrit dvā́.

Discussion

*dwóh₁ is the Proto-Indo-European word for "two," one of the most securely reconstructed items in the PIE lexicon.‌​‍​‍​‌​‍​‌​‍​‌​‍​‍​‍​‍​‌​‌​‌​‌​‍​‍​‌​‌​‍​‌​‌​‌​‌​‍​‍​‌​‌​‌​‌​‌ The numeral shows remarkable stability across all branches.

In Germanic, *dwóh₁ became Proto-Germanic *twō (with *d > t under Grimm's Law), giving Old English twā and modern English "two." Related derivatives include "twin," "twelve" (two left over after counting to ten), "twenty," "twice," "between," and "twilight" (the "two-light" period between day and night).

Latin duo "two" preserved the original *d-, yielding English borrowings like "dual," "double," "dubious" (of two minds), "duplicate," and "duet." Greek δύο (dúo) gave "dyad" and the prefix di- (as in "dilemma").

Sanskrit dvā́ and Avestan dva show the satem branch reflex. Old Church Slavonic dŭva and Lithuanian dù continue the root in Balto-Slavic. Old Irish dá(u) and Welsh dau represent the Celtic branch.

The PIE dual grammatical number — a dedicated inflection for pairs — was built on this root, reflecting its fundamental importance. The dual survives fully in Sanskrit, partially in Greek, and in fossil forms elsewhere.

Notes

Source of "dual", "double", "doubt", "diploma", "between"