A head is an element used to seal the end of a container, isolating its internal and external media, also known as an end cover. 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 an outwardly convex surface, such as hemispherical, elliptical, dish-shaped, and seamless spherical heads. Some gas cylinders use a composite bottom head with the convex surface inward, which ensures strength while also meeting safety requirements.