What tool is commonly used for measuring the angle of a joint?

A protractor or welding angle finder is specifically designed for measuring angles, making it the ideal tool for assessing the angle of a joint in welding applications. This tool allows welders to accurately determine the angle between two pieces of material before starting the welding process, ensuring that the work meets the required specifications and joint design.

A scale ruler is primarily used for measuring lengths rather than angles, which makes it unsuitable for angle measurements needed in welding. A caliper is designed for measuring the dimensions of objects, such as depth, internal, and external diameters, but does not provide the necessary functionality for assessing angles. Similarly, a micrometer is precision instrumentation used for measuring small distances or thicknesses but, like the caliper, is not equipped for measuring angles. Therefore, the protractor or welding angle finder stands out as the correct choice for measuring joint angles in welding.