In general, a bearing is a cylindrical piece of machinery that functions to reduce friction between two flat surfaces in contact. The simplest and most inexpensive form of bearings are plain bearings. Like all bearings, plain bearings are round and minimize points of contact within the connecting part of machinery. Hydrostatic bearings and hydrodynamic bearings are more complex forms of bearings classified as fluid bearings. They consist of rotating and stationary ring-shaped elements which rely on a thin layer of fluid, either gas or liquid, to lubricate the gap. A hydrostatic bearing is named for the constant size of its gap, while a hydrodynamic bearing is named for the varying sizes of its gap.