In the petroleum industry, a chemical process in which an olefin (ethylene, propylene, and so forth) and a hydrocarbon, usually 2-methylpropane, are combined to produce a higher-molecular-weight and higher-carbon-number product. The product has a higher octane rating and is used to improve the quality of gasoline-range fuels. The process was originally developed during World War II to produce high-octane aviation gasoline. Its current main application is in the production of unleaded automotive gasoline.