A cap is an element used to seal the ends of containers to isolate internal and external media, also known as a cover plate. Caps for cylindrical containers are typically rotational shells. They can be categorized by the shape of their surface into convex, conical, flat, and combined shapes. A convex cap refers to a cap with a convex outer surface, such as hemispherical, elliptical, dish-shaped, and seamless spherical caps. Some gas bottles use a combined bottom cap with the convex surface facing inward, which ensures strength as well as meets safety requirements for use.