A Sliding Bevel is a woodworking tool which is used to duplicate an angle.
This may be necessary when trying to miter skirting boards for example, when the room is not square.
The Bevel is loosened by unscrewing the nut or screw which clamps the sliding part of the Bevel (the blade) to the handle (or stock). The handle is then pushed against one of the angles and the blade is moved to rest against the other angle. The nut is then tightened and you are left with an accurate representation of the angle which is easily transferred to skirting or other timbers etc.
In the image below, for example, an internal angle of 70 degrees is shown in the corner of a room. If a shelf were required in this corner it would obviously need to be cut to 70 degrees. There is not always a protractor at hand, but when doing any carpentry, it is always useful to carry a sliding bevel.
The stock is held against one wall, the blade against the other. The nut tightened and the 70 degree angle can then be transferred to the timber which, in turn, can be cut to make the shelf.
The great thing about a sliding bevel is that the same angle can be transferred any number of times so it does not have to be measured every time.