what is the hex key？

A hex key, also known as an Allen key or wrench, is a tool of hexagonal cross-section used to drive bolts and screws that have a hexagonal socket in the head (internal-wrenching hexagon drive).

It is also known as a zeta key which refers to the sixth letter of the Greek alphabet. The term hex-head is sometimes used to refer to this type of drive, but this use is not consistent with its more conventional use referring to external-wrenching hexagons.

In the fastener industry, the terms socket head or hex socket head are generally used for the driven part of the driver-driven pair.