An assembly of valves and fittings used to control the pressure and flow in oil and gas wells. It is located on top of the wellhead and provides the controlling mechanism for the isolation of wells. It has many other functions including permitting the injection of chemicals into wells and pressure relief. Playing an essential role in an emergency shutdown (ESD), a basic form of assembly has several manual gate valves with typically four or five valves being arranged in a crucifix arrangement. The name is derived from its resemblance in both shape and decoration to a Christmas tree.