Antique Joinery Hand Tools
Mortise gauge
The same moment, today
Two pencil marks on a wall rarely line up if they're measured one at a time. A tape measure wobbles a little, or the pencil tip gets dull, and the second line ends up an eighth of an inch off. When two parts have to fit together, measuring them separately almost guarantees they won't match. It's much easier to set a gap once and carry it over than it is to measure twice and hope for the best.
That's the job of a mortise gauge. It has two sharp steel points on a wooden beam, and a sliding fence that locks down with a screw. You don't use numbers or a ruler to set it. You hold the chisel between the points, lock the gap to match the tool, and slide the fence along the wood. The two points scratch both lines at once. Because the gauge doesn't change, the slot on one board and the peg on the other are always the exact same width.
The lesson
When two things need to fit together, don't measure each one separately. Lock the spacing in place and use the same mark for both.
Where this comes from
Peter Nicholson's two teeth
Nobody knows who made the first mortise gauge. The tool was already common when Scottish architect Peter Nicholson wrote about it in London in 1812. In his book, Mechanical Exercises, he explained that a standard gauge only had one tooth to mark a line. The mortise gauge added a second tooth on a small slide. That let a woodworker set the exact gap between the teeth and keep it steady. Decades later, companies like Stanley Rule & Level Co. in New Britain, Connecticut began making them in factories, patenting metal improvements in 1872.
Locking a distance once works better than measuring it every time you cut.
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