Dual can be used to connotate something being both at the same instant (i.e,. being twofold), such as in duality. The duality of a phenonomen is characterized by the fact that some properties can be explained best by one model and others by another model. (see wave-particle duality)

In some languages, in addition to the singular and plural there is also a **dual**, which is used when exactly two people or things are meant.
The plural in these languages is used only for groups greater than two. This is known as the dual grammatical number.

The word **dual** is used in mathematics, as in dual space, dual polyhedron and dual numbers, for example. See Duality.

**Dual** is a brand name for Hifi equipment, mostly known for vinyl record players.

**Dual,** in electronics, refers to two devices or circuits that have mathematical descriptions that are identical except that voltages in one correspond to currents in the other, and vice versa. For example:

- a resistance is the dual of a conductance;
- two resistances in series are the dual of two conductances in parallel;
- a capacitor is the dual of an inductor;
- Kirchoff's current law is the dual of Kirchoff's voltage law.