Plural of “complex”

The plural of complex is complexes.

Why

"Complex" descends from Latin "complexus," meaning something woven or braided together, from "com-" (together) plus "plectere" (to weave). English adopted it first as an adjective, and only later developed the noun senses — a group of connected buildings, or a psychological pattern of ideas and feelings (as in "inferiority complex," a term popularized by early 20th-century psychology). Both noun senses developed within English rather than being carried over from a Latin noun usage, so both take the regular English -es plural.

Unlike "vortex" or "apex," "complex" never had a genuinely Latin noun plural competing for space in English, because the Latin word "complexus" was itself functioning more like a participle/adjective in its original grammar than as the kind of countable third-declension noun that produces an -ices plural pattern in borrowed English vocabulary.

Usage note

"Complexes" covers both major senses without any distinction in form: a city's sports complexes, a person's psychological complexes. Context alone tells the reader which sense is meant.

The psychological sense owes its popularity largely to Alfred Adler and Sigmund Freud's early 20th-century writing, and by the time the term filtered into everyday English from academic psychology, "complexes" was already the established, unquestioned plural — there was never a period of genuine competition with an alternate form. The architectural sense of "complex" — a group of related buildings sharing a purpose, like an office complex or a temple complex — developed independently from the psychological sense, both emerging from the same underlying Latin idea of things "woven together," but along entirely separate paths through English vocabulary, one through construction and urban planning, the other through Vienna's early psychoanalytic circles.

Common real mistakes

Writers occasionally drop the -es and write "complexs," particularly when typing quickly; this word requires the same full "-es" ending as "annex" and "duplex," its two closest relatives among the fully anglicized Latin -ex nouns.

The wider pattern

"Complex" reinforces the same pattern seen in "annex" and "duplex": Latin -ex nouns that developed their English noun sense recently, or through a route (like popular psychology) that never involved classical scholarship directly, uniformly pluralize with -es rather than any Latin-derived ending. Video game and entertainment complexes have become a distinct modern category of the architectural sense, combining arcades, theaters, and dining under one roof, a 21st-century extension of the same basic idea behind older sports and shopping complexes, all sharing the identical regular plural.

Examples

• The city built two new sports complexes on opposite sides of town.
• Her therapist helped her work through several long-standing complexes.
• Both apartment complexes were managed by the same company.
• Psychologists once cataloged dozens of named complexes, though few remain in clinical use today.
• The university added three research complexes to its growing campus.
• Multiple shopping complexes opened along the highway within a few years of each other.
• The novel's characters each carry their own private complexes about family.
• Two military complexes were decommissioned after the base closure.
• Archaeologists uncovered several temple complexes buried beneath the city.
• The report compared energy use across five industrial complexes.
• The retail complex added a new wing to accommodate more stores.
• Both apartment complexes were built by the same developer in the 1980s.
• Her book explores several complexes common among only children.
• The government complex houses three separate agencies under one roof.
• The entertainment complexes downtown attract visitors from across the region.
• Two rival shopping complexes opened within a mile of each other.
• Her psychology practice specializes in treating deep-seated childhood complexes.
• The university built new residential complexes to house incoming students.
• The city's two sports complexes host rival high school teams.
• Her novel explores the complexes that shaped an entire generation.
• The city merged two aging complexes into a single modern facility.
• Her thesis catalogs recurring complexes across a decade of case studies.