A near relation of the try square, the bevel gauge also consists of a handle or stock with an attached tongue (or blade). In the case of the bevel gauge, however, its purpose is to fabricate or replicate pieces that are not square.
In its various forms, this tool is also known as a sliding bevel, angle bevel, bevel square, sliding T-bevel, or adjustable try square. Tongues range from seven inches upward, sometimes to eighteen inches or more. Whatever the length of the blade, the stock is always shorter.
The tongue of the bevel gauge fits into a groove cut into the head of the stock. In most models, a slot cut into the tongue allows further adjustments of the position of tongue and stock.
The tongue is usually made of thin steel and the stock of wood, plastic, or metal. Over the years, many bevel gauges have been made with rosewood or ebony and brass; sometimes, especially in handcrafted models, the tongue material is also wood.
To put the tool to use, hold the stock against an edge with the tongue stretched across the workpiece. The tongue can be shifted to assume any angle between zero and one hundred and eighty degrees. When the bevel gauge is positioned at the angle you desire, use its locking mechanism (usually a bolt and wing nut, sometimes a lever device) to set the tool at that angle.
I've found the bevel gauge to be one of those wonderful tools that belies its simple design. When used properly, this modest little tool makes certain otherwise difficult tasks easy to do well. For example, to match an existing angle, you set the gauge, then relocate the tool onto the stock from which you want to make the companion piece. Then you're just a line and a cut away from matching the angle exactly. Best of all, there's no geometry or arithmetic, just marking and cutting.
The bevel gauge can be set from a square, protractor, or from an existing piece to be duplicated. It can be used to check the bevel or chamfer on a piece by setting it to match the bevel at one position on the workpiece and then sliding the bevel along its length, with the stock held flush against the piece's face. Where light is visible beneath the blade of the bevel gauge, the chamfer is not true.
If you do any restoration work at all, or cut oddball angles, buy a bevel square. I think you'll be surprised how often it proves its worth.