# Types have a cardinality¶

In type theory a type has a cardinality. The cardinality for a given type is the number of possible values. A cardinality can be finite or infinite. A pair of types with the same cardinality will always be isomorphic.

Cardinality for a type $T$ is expressed $|T|$. For example for the type

data Boolean = True | False


Then $|Boolean| = 2$ should be read as the cardinality of Boolean is equal to 2