dwi-plo-
“twofold, doubled”double, diploma, diplomat, duplex
Compound of *dwi- and *plo- giving Latin duplex, English double, diploma, diplomat, duplicate.
Discussion
The PIE form *dwi-plo- (twofold, doubled) is a compound of *dwi- (two, from the cardinal numeral *dwóh₁) with *-plo- (fold, layer — a suffix that creates multiplicative adjectives). The literal meaning is "two-layered" or "folded in two."
Greek diplóos (διπλόος, "twofold, double") is the transparent reflex and the source of the most recognisable derivatives: diploma (δίπλωμα, originally a document folded in two — a letter of introduction or credential folded double for sealing and transport), diplomat (one who carries diplomas, hence one who conducts international negotiations with proper credentials), and diplomacy (the art of official credential-bearing intercourse between states). The semantic journey from "folded paper" to "international relations" is one of the most unexpected in the IE vocabulary.
Latin duplus (double) and its derivatives are equally productive: double (through French), duplicate, duplex (a two-fold dwelling), and duplicity (double-dealing, the state of being two-faced — a moral metaphor from the numerical concept).
The multiplicative suffix *-plo- is itself highly productive in PIE. It appears in *tri-plo- (threefold → Latin triplus → English triple), *kʷetwr̥-plo- (fourfold → Latin quadruplus → English quadruple), and the general pattern extends to any numeral: quintuple, sextuple, centuple. The English suffix -fold (twofold, threefold, manifold) is the native Germanic equivalent of the same pattern, from OE -feald (PGmc *-falþaz), related to the verb fold.
The connection to folding is literal, not metaphorical. *-plo- derives from the PIE root *pel- (to fold, to turn), which also gave Latin plicāre (to fold) → English ply, apply, complicate (folded together), simple (sim-plex, "one-fold"), complex (com-plexus, "folded together"), multiply, and reply (to fold back). The entire vocabulary of multiplication and complexity rests on the image of folding cloth or paper.
The numeral base *dwi- (two) is one of PIE's most stable elements: English two, Latin duo, Greek dýo, Sanskrit dvā́, Lithuanian du, Old Church Slavonic dŭva, and Irish dá all continue *dwóh₁ with minimal change. The stability of low numerals across IE branches — one, two, and three are among the most conservatively inherited words in any IE language — reflects their fundamental cognitive status: these are words learned earliest, used most frequently, and resistant to borrowing or replacement.