Manufacturing diversified by 1914 to printing, engineering, chemicals and clothing manufacture. For example, a company may manufacture cars, and later begin manufacturing trucks. The factory will manufacture firearms for the Indian army. GMP guidelines are not prescriptive instructions on how to manufacture products. A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Plant cells do manufacture cholesterol, but in rather small quantities. It is produced mainly as a byproduct of acrylonitrile manufacture. There are three processes for metallic pipe manufacture. Some plywood is also labeled based on the wood used to manufacture it. Cottage industries in the area including chocolate, pickle manufacture, and carpentry. Dinitrotoluene is the precursor to toluene diisocyanate, which used in the manufacture of polyurethane foam. Many companies manufacture and distribute tennis rackets. DFMA stands for Design for Manufacture and Assembly. Sulfur dioxide is primarily produced for sulfuric acid manufacture (see contact process). Numerous other improvements that made for easier manufacture and installation were continually implemented.