Double-U-groove

You make the double-U groove when you want to weld sheet metal from both sides and still get a proper full penetration weld. You mill or grind the U shape into both sides, leaving a land in the middle. Then you weld it from one side first and then from the other. The advantage is that you need less filler material than with an X-groove and distortion stays within limits because you can work from both sides. It's important that the two U-grooves are clean and symmetrical, otherwise the part will tip over during welding or you won't get a clean full penetration on one side.