18mm Female to 18mm Male Glass Adapter allows you to use a variety of bowls, dropdowns, or other glass accessories on your water pipe or oil rig. Glass Adapters are optional attachents for smoking pipes and water pipes. Glass smoking pipes have three standard joint sizes: 10mm, 14mm and 18mm. Joints also come in different "genders" male or female. Female pieces fit with male pieces and vice versa. Glass Adapters convert one size to a different size, one gender to the other gender, or a combination of size and gender changes. If you find yourself with a bowl or nail that doesn't fit your water pipe, just add one of our glass adapters to make it fit!