production problems (such as blockages). Functionality may be extended further by using the control system on a subsea tree to monitor, measure, and react to sensor outputs on the tree or even down the well bore. The control system attached to the tree controls the downhole safety valve (SCSSV, DHSV, SSSV) while the tree acts as an attachment and conduit means of the control system to the downhole safety valve. Tree complexity has increased over the last few decades. They are frequently manufactured from blocks of steel containing multiple valves rather than being assembled from individual flanged components. This is especially true in subsea applications where the resemblance to Christmas trees no longer exists given the frame and support systems into which the main valve block is integrated. Note that a tree and wellhead are separate pieces of equipment not to be mistaken as the same piece. The Christmas tree is installed on top of the wellhead. A wellhead is used without a Christmas tree during drilling operations, and also for riser tie-back situations that later would have a tree installed at riser top. Wells being produced with rod pumps (pump jacks, nodding donkeys, grasshopper pumps, and so on) frequently do not utilize any tree owing the absence of a pressure-containment requirement. Valves Subsea and surface trees have a large variety of valve configurations and combinations of manual and/or actuated (hydraulic or pneumatic) valves. Examples are identified in API Specifications 6A nd 17D. A basic surface tree consists of two or three manual valves (usually gate valves because of their flow characteristics, i.e. low restriction to the flow of fluid when fully open). A typical sophisticated surface tree will have at least four or five valves, normally arranged in a crucifix type pattern (hence the endurance of the term "Christmas tree"). The two lower valves are called the master valves (upper and lower respectively). Master valves are normally in the fully open position and are never opened or closed when the well is flowing (except in an emergency) to prevent erosion of the valve sealing surfaces. The lower master valve will normally be manually operated, while the upper master valve is often hydraulically actuated, allowing it to be used as a means of remotely shutting in the well in the event of emergency. An actuated wing valve is normally used to shut in the well when flowing, thus preserving the