A jig is any of a large class of tools in woodworking, metalworking, and some other crafts that help to control the location or motion (or both) of a tool. Some types of jigs are also called templates or guides. The primary purpose for a jig is for repeatability and exact duplication of a part for reproduction. An example of a jig is when a key is duplicated, the original is used as a jig so the new key can have the same path as the old one. In the advent of automation and CNC machines, jigs are not required because the tool path is digitally programmed and stored in memory. jigs may be made for reforming plastics, and also for use in reproduction of materials. this also includes templates. see templates for details
Jigs or templates have been known long before the industrial age. There are many types of jigs, and each one is custom-tailored to do a specific job. Many jigs are created because there is a necessity to do so by the tradesmen. Some are purely to increase productivity, to do repetitious activities and to add precision to a job. Because jig design is fundamentally based on logic, it is highly possible that the same jig created in one geographical region was created independently in another region or was created previously in another era and all the creators were unaware of each other's jig yet created it in an almost identical manner.
A jigsaw is called so because it is used to cut arbitrary curves used in jigs.
Drill jigs started falling into disuse with the invention of the jig borer.
Since the widespread penetration of the manufacturing industry by CNC machine tools, in which the servocontrols are capable of moving the tool to the correct location automatically, the need for drill jigs (and for the jobs of the drill press operators who used them) is a fraction of what it previously was.