# dual

/ˈduəɫ/ (US) · /djˈuːəl/ (UK)

Source: https://englishreference.com/word/dual
Updated: 2026-06-19

## Senses

### 1. having two parts

adjective · CEFR B2

having two parts, or having two different purposes at the same time.

For teachers: consisting of two parts, elements, or aspects; often used to describe a single entity with two distinct functions.

- The new laptop has a dual purpose as both a computer and a tablet.
- The candidate holds dual citizenship, which complicates the security clearance process for the high-level government position.

Usage: Usually placed before the noun it modifies.

### 2. grammar

adjective · CEFR C1 · Technical

a special form of a word used in some languages to talk about exactly two people or things.

For teachers: pertaining to a grammatical number that specifically denotes two of a kind, distinct from singular and plural.

- Some ancient languages have a dual form for pairs like eyes or hands.
- While Modern English relies on the plural, Old English utilised a dual number for the first and second person pronouns.

Usage: Used as a technical descriptor for morphological categories.

### 3. mathematics/physics

adjective · CEFR C2 · Academic · Technical

describing a relationship where two things are opposites but also match each other perfectly.

For teachers: relating to duality, where one mathematical structure or physical theory can be translated into another.

- The scientist explained the dual nature of light as both a particle and a wave.
- In projective geometry, every theorem has a dual statement where the roles of points and lines are interchanged.

### 4. the other of a pair

noun · CEFR C2 · Technical

one of two things that form a pair, especially in math or logic.

For teachers: an object, property, or proof that exists in a relationship of duality with another.

- The cube is the dual of the octahedron.
- To solve the optimisation problem, the student first had to derive the dual of the original linear programme.

### 5. to expand a road

verb · CEFR C1 · British English · Technical

to change a road with one lane in each direction into a larger road with two lanes in each direction.

For teachers: to convert a single-carriageway road into a dual carriageway.

- The government plans to dual the main highway to reduce traffic jams.
- The local council has finally secured the funding required to dual the bypass, a project expected to take eighteen months.

Usage: This is a transitive verb, typically used in civil engineering or urban planning contexts.

## Examples

- The car has a dual exhaust system.
- She holds dual citizenship in both Canada and the United Kingdom, allowing her to work freely in both countries.
- The proposed legislation serves a dual purpose: it aims to stimulate economic growth while simultaneously providing a robust framework for environmental protection.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.