Commercial gasoline is a mixture of a large number of different hydrocarbons. Gasoline is produced to meet a host of engine performance specifications and many different compositions are possible. Hence, the exact chemical composition of gasoline is undefined. The performance specification also varies with season, with more volatile blends (due to added butane) during winter, in order to be able to start a cold engine. At the refinery, the composition varies according to the crude oils from which it is produced, the type of processing units present at the refinery, how those units are operated, and which hydrocarbon streams (blendstocks) the refinery opts to use when blending the final product.