Static model aircraft are commonly built using plastic, but wood, metal, card and paper can also be used. Models are sold painted and assembled, painted but not assembled (snap-fit), or unpainted and not assembled. The most popular types of aircraft to model are commercial airliners and military aircraft. Popular aircraft scales are, in order of increasing size: 1:144, 1:87 (also known as HO, or "half-O scale"), 1:72 (the most numerous), 1:48 (known as "O scale"), 1:32, 1:24, 1:16, 1:6, and 1:4. Some European models are available at more metric scales such as 1:50. The highest quality models are made from injection molded plastic or cast resin. Models made from Vacuum formed plastic are generally for the more skilled builder. More inexpensive models are made from heavy paper or card stock. Ready-made die-cast metal models are also very popular. As well as the traditional scales, die-cast models are available in 1:200, 1:250, 1:350, 1:400, 1:500 and 1:600 scale.