Plural of “duplex”

The plural of duplex is duplexes.

Why

"Duplex" comes from Latin "duplex," meaning twofold or double, built from "duo" (two) and "plicare" (to fold). It entered English as an adjective long before it became a common noun for a two-unit residential building in 20th-century American real estate, and by the time that noun sense developed, English had long since stopped importing Latin plurals for new everyday coinages — so "duplexes" took the standard English -es ending from the start.

There has never been a competing Latinate plural like "duplices" in serious circulation for the housing sense of the word, largely because the noun usage is a distinctly American 20th-century development rather than an inherited scholarly term — it names a building type, not a classical concept, and building types in English pluralize regularly.

Usage note

"Duplexes" is used in everyday real estate, zoning, and housing contexts without any competing form; you'll also see it in audio and telecommunications jargon (a "duplex" communication channel), where the plural is equally regular.

Regional usage varies on what exactly a "duplex" is — in some parts of the U.S. it strictly means a single building split into two side-by-side units, while elsewhere it can describe stacked units — but the plural "duplexes" is stable across all these regional senses. Outside real estate, "duplex" also appears in telecommunications as a technical term for a communication mode allowing signals to travel in both directions at once ("full-duplex" versus "half-duplex" systems), and even in that engineering context, the plural "duplexes" (referring to multiple such systems or channels) stays fully regular, with no Latin alternative ever proposed.

Common real mistakes

Because the word ends in the same -ex cluster as "vortex" and "apex," some writers assume it needs a Latin-style plural and reach for an invented form like "duplices" — this is incorrect; "duplexes" is the only standard plural.

The wider pattern

"Duplex" is a useful contrast within the -ex family precisely because it shows that ending in -ex doesn't automatically invite Latin plural competition; a word's actual history — whether it passed through classical scholarship or was coined recently in ordinary English — is what determines whether a Latinate plural ever had a chance to take hold. The rise of house-hacking and multi-generational living arrangements has renewed interest in duplexes in many American housing markets, with several cities loosening zoning restrictions specifically to allow more duplexes to be built as a way to add housing density without dramatically changing a neighborhood's character.

Examples

• The couple bought one of two duplexes on the corner lot.
• Several duplexes on the block were built in the same year.
• The city rezoned the street to allow duplexes alongside single-family homes.
• Both duplexes shared a single driveway between them.
• The developer converted three old houses into duplexes.
• Rent for the duplexes rose faster than for nearby apartments.
• The neighborhood's duplexes were originally built for factory workers in the 1920s.
• Two identical duplexes stood at either end of the cul-de-sac.
• Inspectors flagged wiring issues in several of the older duplexes.
• The listing included photos of all four duplexes for sale together.
• Two identical duplexes were sold within the same week.
• The realtor listed five duplexes suitable for first-time investors.
• City inspectors flagged outdated wiring in several of the old duplexes.
• Both duplexes on the corner shared a single water line.
• The city council debated allowing more duplexes in single-family zones.
• Both duplexes on the street were renovated by the same contractor.
• Several new duplexes were built to address the local housing shortage.
• The investor specifically targets duplexes for their rental income potential.
• The city approved permits for six new duplexes this year.
• Both duplexes were sold to the same investment group.