Dual means having two parts, two purposes, or relating to two things. A dual-purpose room might serve as both a home office and a guest bedroom. A dual carriageway is a road with two separate roadways for traffic going in opposite directions. When something is dual, it exists or functions in two ways at once.

In mathematics, dual numbers are pairs of numbers used together in calculations. Some vehicles have dual fuel systems that can run on either gasoline or natural gas.

People often confuse dual with duel, which sounds exactly the same but means a formal fight between two people. Remember: dual with an “a” means two of something, while duel with an “e” means a contest between two opponents.

You might have a dual role on your soccer team as both a defender and midfielder. Someone with dual citizenship holds passports from two different countries. When a tool has a dual function, like a hammer that also pulls nails, it combines two useful purposes in one object. The word emphasizes how two elements work together or exist simultaneously rather than separately.