Sliding Bevel (Bevel Gauge)

A sliding bevel (also bevel gauge) has a blade that pivots and locks at any angle relative to its handle, set by loosening a wing nut or lever, positioning the blade against an existing angle, and locking it there. Unlike a fixed square, it captures whatever angle is actually in front of you -- an out-of-square wall, an odd furniture joint, a replicated part -- rather than one you have to calculate and dial in on a protractor.

Once set, the tool transfers that exact angle to a workpiece for marking a cut line, or checks a cut against the original angle after the fact. For dovetail joint layout specifically, a fixed-ratio dovetail marker is more common than a general sliding bevel, since the angle needs to stay perfectly consistent across many repeated cuts.