Sliding Bevels are a type of hand tool used for measuring and marking angles. They consist of a metal blade with a handle attached at one end, and a movable arm that slides along the blade. The bevel can be used to transfer angles from one surface to
From Wikipedia
A sliding T bevel, also known as a bevel gauge or false square is an adjustable gauge for setting and transferring angles. Different from the square, which is fixed and can only set a 90° angle, the sliding T bevel can set any angle and transfer it on another piece.