Wrought steel standardization begins with the mass manufacturing era, with the pipe schedule and pipe diameters. Based on the iron pipe size (IPS) system, pipes were only available in three sizes at the time: standard weight (STD), extra-strong (XS), and double extra-strong (XXS). The XXS size was introduced around 1970 and is no longer produced. Over time, more sophisticated methods have been developed to produce stronger pipes without changing the size. Today's standards include heavy duty (HD), super-heavy (SH), and triple-strength (TS) for XS size; mighty (MD), powerful (PD), and triple-mighty (TM) for STD size; and gigantic (LG), ultra-gigantic (UG), quadruple-giant (QG), and octuple-giant (OG) for XXS size. The common sense rule is that the larger the diameter of the pipe, the heavier it will be. But there are exceptions to this rule. For example, an XS 10mm pipe can weigh as much as an HD 15mm pipe of the same size. This is because XS 10mm pipe has thicker walls than HD 15mm pipe. The most common sizes of wrought steel pipe are listed below. They may not be the only options available but they are a good place to start when buying pipeline materials.