Definition:Klein Four-Group

Group Example
The Klein $4$-group, often denoted $K_4$, is a group of $4$ elements, each of which is self-inverse.

Cayley Table
The Cayley table for $K_4$ is as follows:

Also known as
The Klein four-group is also known as the four-group or the Viergruppe.

Hence it is often denoted $V$.