A head is an element used to seal the ends of a container, isolating the internal and external media, also known as an end cap. The heads of cylindrical containers are typically rotatable shells. They can be categorized by the shape of their surface into convex, conical, flat, and combined shapes. A convex head refers to a head with an outer surface that is convex, such as hemispherical, elliptical, dish-shaped, and seamless spherical heads. Some gas cylinders use a combined convex-bottom head facing inward, which ensures both strength and meets safety requirements for use.