An orifice is any opening, mouth, hole, or vent, as in a pipe, a plate, or a body. It can be an anatomical orifice, such as the mouth or a bodily orifice. Orifices can be used to control pressure or flow, measure flow rates, and meter fluid flow in pipes and reservoirs. The orifice plate is a metal disk with a concentric hole in it, which is inserted into the pipe carrying the flowing fluid. Orifice flanges are used to measure the flow of product through the piping system, and when the orifice plate is installed, there is a restriction of flow creating a differential pressure that is used to measure the flow rate of liquids, steam, or gases. The coefficients of velocity and discharge are necessary to accurately predict flow rates from orifices.