duplex

Etymology

It is borrowed from Latin duplex (“twofold, double”),^{[2][3][4][5][6]} formed from duo (“two”)^[2][3][4] and the element -plex (“-fold”).^[7][8][6] The second part is explained in different ways: the OED, the SOED and the ODE connect it with plicō (“to fold”),^[2][9][10] Etymonline with Proto-Indo-European *plek- (“to plait”),^[3] and Barnhart with Proto-Indo-European *plāk-, through a Greek word for a flat surface.^[4] The Latin word is cognate with Ancient Greek δίπλαξ (díplax, “twofold, double”).^[6][4][7] Etymonline traces duo to Proto-Indo-European *dwo- (“two”).^[3][5] The OED notes that the word was not yet in Webster's dictionary of 1828.^[2]

Earliest recorded meaning

Composed of two parts or elements; twofold: 1817, in Peacock's Melincourt.^[2][4][3] Merriam-Webster dates the adjective to 1567,^[7][11] the SOED and the ODE to the mid-16th century,^[9][10] and Dictionary.com to 1810–20.^[12]

Semantic evolution

Technical uses for machines and devices with two parts working together are recorded from 1851, in the catalogue of the Great Exhibition,^[2] as in the duplex escapement of a clock or watch.^[9] In telegraphy, from 1873, a duplex system sent two messages along the same wire at once,^[2] and the verb duplex, to arrange a wire or cable in this way, followed in 1880,^[2] though Merriam-Webster dates the transitive verb to 1833.^[7] Later the word was applied to paper made of two layers or coloured differently on each side (1901), to eyes with pigment on both surfaces of the iris (1908), in genetics to a polyploid with a dominant allele present twice (1923), and in biochemistry to two linked polynucleotide strands (1969).^[2] In metallurgy a duplex process makes steel in two successive furnaces.^[9][7] The noun, a house divided into two dwellings or a flat on two floors, began in American English in 1922, in an Oklahoma newspaper,^[2][3] originally in the phrases duplex house and duplex apartment;^[4] Collins still labels it US and Canadian.^[8]

Timeline

• 1817 – composed of two parts; twofold.^[2][4][3]
• 1851 – in technical uses, of devices with paired parts.^[2]
• 1873 – in telegraphy, of a system sending two messages on one wire at once.^[2]
• 1880 – verb, to make a wire or cable duplex.^[2] Merriam-Webster dates the verb to 1833.^[7]
• 1901 – of paper made of two layers.^[2]
• 1922 – noun, a house divided into two dwellings; a flat on two floors.^[2][4][7]
• 1923 – in genetics, having a dominant allele represented twice.^[2]
• 1963 – noun, a double-stranded polynucleotide molecule.^[2]

Meanings

Adjective

1. Having two parts or elements; twofold, double.^{[13][7][8][2]}
2. Of machinery: having two identical units that can work at the same time or independently.^[13][8][7]
3. Of a telecommunications channel: allowing transmission in both directions at the same time.^{[13][8][11][9]}
4. (biochemistry) Having two complementary polynucleotide strands, as DNA does.^[11][2]
5. Of paper or board: made of two layers, or coloured or finished differently on each side.^[2][7][9]

Noun

1. A house divided into two separate homes, usually with their own entrances.^[13][7][2]
2. A duplex apartment, one with rooms on two floors.^[13][7][2]
3. (biochemistry) A double-stranded molecule of DNA, or of DNA and RNA.^[7][8][2]

Verb

1. To make duplex, especially to arrange a telegraph line or cable so that two messages can travel at once.^[7][2][9]

Word family

• duplexity
• heteroduplex
• half-duplex

Related words

• diplex
• quadruplex
• triplex
• double
• duplicity
• multiplex
• simplex
• duplicate
• complex
• dual
• two

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k l m n o p q r s t u v w x y Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e f g Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
4. ^ a b c d e f g h Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
5. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
6. ^ a b c Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
7. ^ a b c d e f g h i j k l m n Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
8. ^ a b c d e f Collins English Dictionary (2023), Glasgow: HarperCollins
9. ^ a b c d e f g Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
10. ^ a b Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
11. ^ a b c Merriam-Webster's Collegiate Dictionary (2003), 11th ed., Springfield, Mass.: Merriam-Webster
12. ^ Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
13. ^ a b c d e The American Heritage Dictionary of the English Language (1992), 3rd ed., Boston: Houghton Mifflin