The Double Cup is a design made by Trevor Allen Huber(Trevsglass) a few years ago that was never put into distribution until just recently. The design has a little cup extending out from the center surrounded by exterior walls hence, the Double Cup. The tower in the middle has slits on its side where the vapor is pulled down into your rig. The tower sits below the top of the exterior walls so that no vapor can escape while in use. Second quality nails are available at half price. Second quality nails have cosmetic defects (bubbles, scratches, inconsistencies, etc.) that do not affect function.