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