The small objects that are used in packed columns and packed beds. They provide a high surface area per unit volume allowing intimate contact between a rising vapour or gas with a descending liquid. There are many types commonly used, such as saddles and rings, and they are made from plastics, glass, metals, and ceramics that are inert and unreactive to the substances in which they are in contact. They may be arranged in packed columns either in a structured form or randomly, and sit on a support plate designed to take their bulk weight. They are required to allow good passage of liquid and vapour and offer a low pressure drop across the entire packing. The packing fraction is the volume taken by the number of particles within a given space or volume.