A cap is an element used to seal the end of a container, isolating its internal and external media, also known as an end cover. Caps for cylindrical containers are typically rotating shells. They can be classified by the shape of their surface into convex, conical, flat, and composite shapes. A convex cap refers to a cap with an outer surface that is convex, such as hemispherical, elliptical, dish-shaped, and seamless spherical caps. Some gas cylinders use a composite bottom cap with an inward-facing convex shape, which ensures strength while meeting safety requirements for use.