FORGINGS ARE CLASSIFIED ACCORDING TO THE production method and fall into five major headings: Open Die; Closed or Impression Die; Rotary, Ring Rolling, and Extrusion. Further subdivision comes from the types of equipment used to make the forgings: Hammer (steam, hydraulic, or mechanical); Press (steam, hydraulic, multi-directional, or mechanical). For some specialized applications the boundary between open and closed die forging can be blurred as in the use of split dies for valve body manufacture or in the manufacture of wrought locomotive and rail rolling stock wheels.