A head is an element used to seal the ends of a container, isolating its internal and external media, also known as an end cap. The heads of 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 head refers to a head with a convex outer surface, such as hemispherical, elliptical, dish-shaped, and seamless spherical heads. Some gas cylinders use a composite bottom head with the convex surface facing inward, which ensures both strength and meets safety requirements for use.