The wall cloud is a low-hanging, rotating feature below the base of the thunderstorm updraft. "Wall" is probably not a good descriptive word for it, but an early and better characterization, "pedestal", never caught on. A wall cloud often precedes the formation of a tornado. Here, the tornado on the left is roping out as the wall cloud on the right organizes further and strengthens. Out of the shaggy cloud, a new tornado will appear.