chuck key is a tool used to tighten and loosen the chuck part of
a drill in order to hold or release a drill bit.
Chuck keys come in a variety of sizes, but the most common are 6,
8 ,10 and 13mm sizes. The size denotes the diameter of the chuck
key at the point just below the gear head.
Many modern drills, especially cordless drills, don't require a
chuck key as they have a fast-release mechanism which is hand operated.