V Glossary Terms - V Visual Key Control (VKC) – A system of stamping all keys and the plug face of all lock cylinders with standard key symbols for identification purposes. Regular Yale key change numbers are not stamped on the keys. Other key and cylinder stamping arrangements are available, but are not considered Visual Key Control. Volt A unit of electromotive force. Voltage The term used to designate electrical pressure that exist between two points and is capable of producing a flow of current when a closed circuit is connected between the two points.