Tubing is a pipe or hollow cylinder for the conveyance of fluids (liquids or gases). The terms "pipe" and "tubing" are almost interchangeable, although minor distinctions exist — generally, "tubing" implies tighter engineering requirements than "pipe". Both "pipe" and "tube" imply a level of rigidity and permanence, whereas a "hose" is usually portable and flexible. Tubing and pipe may be specified by standard pipe size designations, e.g., nominal pipe size, or by nominal outside or inside diameter and/or wall thickness. The actual dimensions of pipe are usually not the nominal dimensions: A "1-inch" pipe will not actually measure 1 inch in either outside or inside diameter, whereas many types of tubing are specified by actual inside diameter, outside diameter, and/or wall thickness. There are many industry and government standards for pipe and tubing.