Drug nomenclature is the systematic naming of drugs, especially pharmaceutical drugs. In the majority of circumstances, drugs have 3 types of names: chemical names, the most important of which is the IUPAC name; generic or nonproprietary names, the most important of which are the International Nonproprietary Names (INNs); and trade names, which are brand names. Under the INN system, generic names for drugs are constructed out of affixes and stems that classify the drugs into useful categories while keeping related names distinguishable. A marketed drug might also have a company code or compound code.