Product of Cardinals is Commutative

Theorem
Let $\mathbf a$ and $\mathbf b$ be cardinals.

Then:
 * $\mathbf a \mathbf b = \mathbf b \mathbf a$

where $\mathbf a \mathbf b$ denotes the product of $\mathbf a$ and $\mathbf b$.

Proof
Let $\mathbf a = \card A$ and $\mathbf b = \card B$ for some sets $A$ and $B$.

Then: