Jigs and fixtures are production-workholding devices used to manufacture duplicate parts accurately. A jig is a special device that holds, supports, or is placed on a part to be maintained. It is a production tool that guides the cutting tool as the operation is performed. A fixture locates, holds, and supports the work securely so the required machining operations can be performed. Set blocks and feeler or thickness gauges are used with fixtures to reference the cutter to the workpiece. Jigs are divided into two general classes: boring jigs and drill jigs. Boring jigs are used to bore holes that are too large to drill. Drill jigs are used to drill, ream, tap, counterbore, or reverse countersink. Drill jigs are divided into two general types, open and closed. Open jigs are for simple operations where work is done on only one side of the part. Closed jigs are used for parts that must be machined on more than one side.