Even though they do not have threaded exterior surfaces, the porcelain bulb holders can still have mounting rings and lampshade frames screwed onto them. To do this, you use a voltage tester or a thin screwdriver to inwardly bend the brass tabs that press on the interior wall of the bulb holder while at the same time turning the bottom part of the holder. You then guide the upper part of the holder through the locking ring and screw on the lower part again. Because the inner wall of the bulb holder has serrations you must turn as many times as you can until the ring cannot go any farther and simultaneously the anti-rotation device can be heard to click into place.