# Dual¶

In maths, a concepts `dual`

is a 1-1 relationship between that concept and another. It is sometimes an inversion. So A \rightarrow B's dual might be A \leftarrow B. This is quite a general idea though - it's effectively a two-way relationship between two concepts.

## Examples¶

- Set Theory: the empty set and the singleton set

## Backlinks¶

- The singleton set
- Unit Type
- It is related to the singleton set from set theory and dual to the void type

- Void Type
- It's dual is the unit type. It corresponds to the empty set from set theory.

## Backlinks¶

- The singleton set
- Void Type
- In type theory, the void type is a type with a cardinality of 0. It is therefore impossible to construct. It's dual is the unit type. It corresponds to the empty set from set theory.

- Dual
- It's dual is the unit type. It corresponds to the empty set from set theory.

- Unit Type
- It is related to the singleton set from set theory and dual to the void type