I call this gizmo my adjustable story board.
The tongue can move in and out and is set in place with the screw.
I use it mostly for inside measure.
The measurement can then be transferred directly to a saw for a very precise cut. It can set the blade-to-fence distance on a table saw, as it is doing here...
Or it can set the blade-to-stop block distance on a radial arm or miter saw.
