Nomenclature
The term "hex key" has various synonyms. Some are explained by the geographical and commercial history of the drive type's development (as detailed in the history section); these include Allen, Unbrako, and Inbus key or wrench. The synonym zeta key or wrench 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. A less common synonym is female hex.
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