In a dab rig, the nail is used for heating the concentrate to the point where it produces vapor. They come in a couple of different styles, including domed and domeless. A domeless nail allows cannabis users to consume concentrates easily and efficiently.
As opposed to a domed nail, the domeless nail boasts a larger size, which means that a larger amount of cannabis concentrate can be used. Ideally, dome or not, the nail should heat up quickly, and should retain heat for a decent period of time.
The lack of a dome also makes it easier to heat the concentrate. However, without the protective dome that other nails feature, the concentrate in the domeless nail also quickly loses heat. Some users place a carb cap on the domeless nail to prevent heat loss.