The word "duplex" entered English in the 1560s directly from Latin duplex (twofold, double), which combines duo (two) with the element -plex, related to plicare (to fold) or the PIE root *plek- (to fold, to plait). Something duplex is literally "folded in two" or "two-layered," and the word has been applied to a remarkable range of two-part structures across several centuries.
The -plex element constitutes one of Latin's most productive combining forms. Simplex (one-fold, single), duplex (two-fold), triplex (three-fold), multiplex (many-fold), and complex (folded together) all use the same formative, creating a systematic vocabulary for describing levels of structural or conceptual layering. English adopted the entire series, and "multiplex" has become particularly prominent as the name for multi-screen cinema complexes.
The broader root *plek- (to fold) generated an enormous English word family through Latin plicare. "Apply" (fold toward), "comply" (fold with), "imply" (fold into), "reply" (fold back), "supply" (fold up from beneath), "complicate" (fold together until tangled), "explicit" (unfolded, made clear), "implicit" (folded in, implied), and "plywood" (layers folded together) all descend from this single root. The metaphorical richness of folding — bringing things together, layering, concealing, revealing — made *plek- one of the most versatile roots in the Indo-European heritage.
In American real estate, "duplex" designates a building divided into two separate dwelling units — either side by side (sharing a common wall) or stacked (one unit above the other). This usage emerged in the 19th century and has become standard in American housing vocabulary. British English uses "semi-detached" for side-by-side duplexes, a term that describes the physical relationship of the two halves rather than the abstract concept of doubleness.
In telecommunications, "duplex" describes a system capable of transmitting in both directions simultaneously — as opposed to "simplex" (one direction only) or "half-duplex" (both directions, but only one at a time). A telephone conversation is full duplex: both parties can speak and hear simultaneously. This technical usage preserves the Latin meaning with precision: a duplex system has two channels operating in parallel.
The DNA double helix is sometimes described as a duplex structure in molecular biology, referring to the two complementary strands that wind around each other. This usage extends the architectural metaphor to the molecular level: two strands folded together into a single functional structure.