A hex key, also known as a hex wrench or Allen wrench, is a tool with a hexagonal cross section. It is used to drive bolts and screws that have hexagonal sockets in the heads. This small and light tool provides extra length and leverage which makes bolts and screws easier to drive in or remove.
A T-handle key, a type of hex key, has a T-shaped top section that is easier for you to grip while turning.