Socket weld elbow is also welded to the pipe and fittings end. Unlike the butt weld elbow, the socket weld elbow has a trapezoidal area at the end. We can insert the pipe end into this area (the diameter of the SW elbow is matched with the outer diameter of the pipe), then do welding work to connect them together. Therefore, the actual diameter of the SW elbow end is greater than the elbow diameter.http://www.zhijusteel.com/elbow/90-degree-elbow/