A marking gauge is one of the most essential tools in woodworking, used to mark out lines parallel to a reference edge or surface. It’s preferred over marking by pencil, because it is more precise and often also easier to use.
Practically every cutting, planing, or chiselling task is preceded by marking with a marking gauge, making it probably the most utilized tool of all in woodworking.
The marking line is scribed into the wood by a pin, knife or wheel attached to a beam. The configurable headstock ensures that the line is parallel to the reference edge or surface.
The marking gauge in this photo is an example of one with a wheel, offerred by Canadian woodworking tools manufacturer Veritas. Many woodworkers make their own marking gauge.