Which tool is commonly used to measure the angle of a joint?

The protractor is primarily designed for measuring angles, making it the most appropriate tool for assessing the angle of a joint. It has a semi-circular or circular design with degree markings that allow for precise angle measurement. When working with joints in ironwork, ensuring that the angle is correct is crucial for structural integrity and proper alignment.

Calipers are used for measuring dimensions, like the thickness or diameter of an object, but they do not provide angle measurements. Measuring tapes are essential for getting linear measurements but are not suitable for gauging angles. Squares, while useful for making straight lines and right angles, do not offer the versatility of measuring a range of angles like a protractor does. Therefore, the protractor stands out as the most effective tool for measuring joint angles in ironworking contexts.