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