This easy-to-manufacture tool is designed to measure the depth of a groove or hole and to mark its sections that do not correspond to a given depth.
To make such a tool you need: wooden block, pencil, ruler, drilling machine, dowel.
First you need to mark the center on one side of the bar. Then drill a 7 mm hole in the center.
Next, you need to drill a hole in the side of the bar. From the edge of the drilled hole, the master draws a line to the side. On this line will be the center of the new hole.
The last thing to do is a wedge of a wooden dowel. It will be inserted into the side hole and fix the pencil.
Basically, the master uses this tool for carpentry. Using it is very easy to check the depth of the groove. You need to set the pencil to the desired length and wedge with a dowel. Then the master simply puts the bar on the processed lumber so that the pencil fell into the groove. If you draw a bar along the plane, the pencil will leave a mark in those places in the groove that are less than the specified depth.
More details can be seen in the video.