The term xylene or xylol refers to a mixture of three structural isomers of the aromatic hydrocarbon dimethylbenzene. Xylene is a clear, colorless, sweet-smelling liquid that is very flammable. It is usually refined from crude oil in a process called alkylation. It is also produced as a by-product from coal carbonisation derived from coke ovens, extracted from crude benzole from gas, or by dehydrocyclodimerization and methylating of toluene and benzene. It is also manufactured from reformate.