A head is a component used to seal the ends of containers, isolating the internal and external media, also known as a cap. The heads of cylindrical containers are typically revolving 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 meeting safety requirements for use.