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 (i.e., a "1-inch" pipe will not actually measure 1" in either outside or inside diameter) — whereas many types of tubing are specified by actual I.D., O.D., and/or wall thickness. There are many industry and government standards for pipe and tubing.