A marking gauge is a woodworking tool that helps make exact measurements for cutting. Consisting of three simple parts, it resembles a slide ruler but is set apart by its marking pin and locking mechanism. There are four distinct varieties of marking gauges that are used in woodworking shops: traditional, mortise, cutting and panel.
A marking gauge usually is constructed from wood or metal, but both types use the same basic design. A marking gauge always has a central arm that operates like a ruler. The arm normally is a square piece of wood or a large metal rod — rarely is it flat — but it does have measurement points ingrained into the material. The head is a a piece of wood or metal that fits around the arm and can slide up and down the arm, also locking into place wherever necessary using a clamp or wedge. Finally, all marking gauges have a marking device at the end of the arm, either a pin, a pen or a pencil.
A marking gauge operates simply enough for novice woodworkers to understand immediately. The head is laid flush against an edge of the wood that needs to be measured, and it is unlocked. The arm is then slid up or down until the measuring pin is in the position to make a mark. The head is locked into place, and the user can press the metal pin into the wood for a quick mark or, if the pencil or pen is inserted, can run the head gently along the edge to create a straight line for cutting.
The marking gauge has been used in wood shops for centuries and has evolved over time into four different types: traditional, mortise, cutting and panel. The traditional marking gauge consists of the basic design and is used for marking lines. A mortise marking gauge has the same basic setup as a traditional gauge, except that it has two marking pins at the end of the arm. A cutting gauge has the same arm and head setup, but it has a sharp knife in the pin position in order to cut across the grain. A panel gauge is an enlarged version of the traditional gauge and is several feet long, but it is mostly outdated because a table saw can perform long, straight cuts without any need for a panel gauge.