A woodworking jig is a device used to control the blade of a saw or the motion of another tool that will cut or otherwise alter wood. Simple jigs can be made from wood, and they can be custom-built for specific woodworking functions; a person can also buy a prefabricated woodworking jig made from wood, plastic, or even metal. The purpose of this guide is to allow a woodworker to make the same cuts repeatedly on different pieces of wood accurately and quickly. Another function of the jig is to steady the piece of wood being cut to prevent excess cutting or inaccurate cuts.
The size, shape, and function of a woodworking jig will vary significantly according to how it is being used and with what tool it is being used. A jig used with a drill, for example, will have a different structure than one used with a saw, since these tools function very differently and the user will need to move the tools in different ways. The woodworking jig may be designed to keep the blade of a saw straight while cutting, and it may also be designed to stop the saw from moving forward at a certain point; this is a good way to cut angles accurately.
A jig designed to work with a drill might lie on top of a piece of wood to be cut. At one or more locations on the jig, holes may be pre-drilled to indicate where on the new woodworking piece the drill must cut. If the pieces of wood being worked are all of a uniform size, the jig can also be made a specific size that will fit snugly over each wood piece. Once in place, the woodworking jig can be used to indicate where cuts need to be made, eliminating the need for the user to make measurements and guess where to position the wood.
In many cases, the woodworking jig itself is used in conjunction with clamps or vises that keep the jig securely fastened to the piece of wood being cut. This is necessary to prevent movement during the cutting process; the tools sliding over the jig can cause it to move, thereby making the cut inaccurate and defeating the purpose of using the woodworking jig in the first place. A jig may be designed to be used with specific clamps, though in many situations, generic C-clamps can be used to secure the guide in place.