dual

alternative abbreviation du.

1. not comparableCharacterized by having two (usually equivalent) components.

   a dual-motor vehicle

   Synonyms: double, twin

2. not comparablePertaining to two, pertaining to a pair of.

   dual citizenship

   3 more examples

   dual engine failure

   Both Deborah and Samuel held dual roles as judges and prophets.

   2020, Grace Ying May, “Women Disciplining Men: A Biblical Pattern of Leadership”, in Aída Besançon Spencer, William David Spencer, editors, Christian Egalitarian Leadership: Empowering the Whole Church According to the Scriptures, page 48

   And some Asian countries are tightening their immigration laws. Japan reinforced its strict stance in January when a court upheld the country’s ban against dual citizenship, rejecting a lawsuit filed by Japanese citizens living in Europe.

   2021 March 15, Jessie Yeung, “These Asian countries are giving dual citizens an ultimatum on nationality – and loyalty”, in CNN

   Synonyms: double, duplicate, double, duplex, duple, duplicate, twissel, twofold

3. not comparablegrammarPertaining to a grammatical number in certain languages that refers to two of something, such as a pair of shoes.

   Modern Arabic displays a dual number, as did Homeric Greek.

4. not comparablemathematicsExhibiting duality.
5. not comparablelinear algebraBeing the space of all linear functionals of (some other space).

   Accordingly, a hyperplane in the sample space is dual to a subspace in the variable space.

   2012, Doug Fisher, Hans-J. Lenz, Learning from Data: Artificial Intelligence and Statistics V, Springer Science & Business Media, →ISBN, page 81

6. not comparablecategory theoryBeing the dual of some other category; containing the same objects but with source and target reversed for all morphisms.

   Every category is dual to its own dual, so if a statement holds in all categories so does its dual.

   1992, Colin McLarty, Elementary Categories, Elementary Toposes, Clarendon Press, →ISBN, page 77

   Synonyms: opposite

plural duals · alternative abbreviation du.

1. Of an item that is one of a pair, the other item in the pair.
2. geometryOf a regular polyhedron with V vertices and F faces, the regular polyhedron having F vertices and V faces.

   The octahedron is the dual of the cube. The icosahedron is the dual of the dodecahedron.

3. grammarThe dual number.
4. mathematicsOf a vector in an inner product space, the linear functional corresponding to taking the inner product with that vector. The set of all duals is a vector space called the dual space.
5. governmentA head-to-head match or meet between two teams, such as two high schools or colleges.

   But Dave agreed to participate in the Bulgarian dual, and that trapped me into having to wrestle, too.

   2014 November 18, Mark Schultz, David Thomas, Foxcatcher: The True Story of My Brother's Murder, John du Pont's Madness, and the Quest for Olympic Gold, Penguin, →ISBN

   2 more examples

   Wrestling duals and meets take place at the Wisconsin Field House. There are no advance single event ticket sales.

   2010 August 3, Kevin Revolinski, Insiders' Guide® to Madison, WI, Rowman & Littlefield, →ISBN, page 137

   Early in 2021, she became the first female to compete and win a match at the Ter-Hark Cup wrestling dual, helping her squad to victory.

   2023 October 19, Jason Norman, American Women in Amateur Wrestling, 2000-2022, McFarland, →ISBN, page 203

present singular third person duals · US participle present dualing · UK participle present dualling · US participle past dualed · US past dualed · UK participle past dualled · UK past dualled · alternative abbreviation du.

1. transitiveTo convert from single to dual; specifically, to convert a single-carriageway road to a dual carriageway.

   I have to declare an interest and I do so with some ambivalence because if the road is dualled it is likely to take half of my front garden.

   1994, Great Britain. Parliament. House of Commons, Parliamentary Debates

   2 more examples

   The power generation and propulsion systems are dualled to accommodate component failure and maintain propulsion at reduced speed should any part of one system be lost.

   2006, David Lowe, Intermodal Freight Transport, page 163

   The investment will allow Nexus to increase service frequencies, reduce journey times, and improve reliability by dualling three sections of line between Pelaw and South Shields.

   2021 September 22, “Network News: Nexus increases Tyne and Wear Metro train order to 46”, in RAIL, number 940, page 23