Stud welding is a form of welding whereby a metallic fastener, e.g. a rod or earth terminal, is fed from a feeder and fused to the surface metal. This process is primarily used to join metal pins, small bolts or other small components to sheet metal. Examples include pins with an external screw thread to which a product can be attached without being visible from the reverse side. This has major advantages for watertight or gastight sealing purposes. Designers also often prefer this technique, as it does not detract from their designs.