All of the blades in our metric feeler gauge sets are true metric sizes and are made of high carbon steel. All thicknesses are measured in millimeters and sizes are marked on each blade for easy identification. Each gauge set comes assembled in a U shaped metal holder to protect the blades when not in use. The user can "fan" out the blades for easy selection, and then return the unused gauges back into the holder. The screw and locknut can be tightened to secure the blades in working position.