The marking gauge is made to help you make precise lines parallel to the edge of a board. An average wood gauge is about 8 inches long, having a scribing spur near one end. A sliding head can be moved through the body and clamped using a thumbscrew at any point. In choosing a gauge, look for one created from hardwood protected with a brass head or metal. A special double-bar gauge having two spurs is made for marking off a mortise. The gauge is set once, and the spur on each side is utilized in turn to show themortise area.
Lay the gauge on the wood to be marked in a way that the spur is at draw cut angle to the work scribed on. Grip the gauge with the fingers over and around the head, the thumb held out along the side of the beam.
Operate the spur by means of a slightly rotary pressure of the gauge. Having the gauge at a slight angle, with the spur pointing away from the direction in which the gauge would be moved, create a line by pushing the gauge using pressure in two directions—forward to mark the line, and sideways to keep the head tightly on the guiding edge.
Care should be taken so that the spur will not break.When the gauge isn't used, move the head along the beam till it rests on the spur and lock it there using the thumbscrew. A file is used to sharpen the spur. The spur must project about 1/16 or 3/32 inch. Don't tighten screw too much. It would wear out the thread in the wood.
If the mark will be made more than 6 inches from the edge of the board, a different type of gauge - the panel gauge - is used. This is just a larger version of the everyday marking gauge. A good alternate is a set of trammel points for long gauges.
When no gauge is in hand, fingers could be used as a guide. Just set a pencil between two writing fingers having the point at the proper length and slide the knuckles along the edge of the board at a steady angle. Even more precise is the use of a zig-zag rule held securely with the left hand at the board edge and a pencil held with the right hand at the end of the zig-zag rule. Most marking gauges are calibrated to 1/16 inch, but some don't have markings at all and have to be set using a ruler.