Pipe is a tube or hollow cyliner for the conveyance of fluid, gas and sometimes other materials. The terms 'pipe' and 'tubing' are almost interchangeable. 'Pipe' is generally specified by the internal diameter (ID) whereas 'tube' is usually defined by the outside diameter (OD) but may be specified by any combination of dimensions (OD, ID, wall thickness), depending upon which are considered the most important to the designer. 'Tube' is often made to custom sizes and may often have more specific sizes and tolerances than pipe. Also the term "tubing" can be applied to tubes of a non-cylindrical nature (i.e. square tubing). The term 'tubing' is more widely used in the USA and 'pipe' elsewhere in the world.