A head is an element used to seal the end of a container, isolating its internal and external media, also known as a cap. The heads of cylindrical containers are typically rotating shells. They can be categorized by the shape of their surface into dome-shaped, conical, flat, and composite shapes. A dome-shaped head refers to a head with an outward 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 meeting safety requirements for use.