Dienes are prepared from the same reactions that form ordinary alkenes. The two most common methods are the dehydration of diols (dihydroxy alkanes) and the dehydrohalogenation of dihalides (dihaloalkanes). The generation of either an isolated or conjugated system depends on the structure of the original reactants. Vicinal diols, which have two hydroxyl groups on adjacent carbon atoms, and vicinal dihalides, which have halogen substituents on adjacent carbons, always become conjugated systems in elimination reactions. Other reactant configurations can lead to products that include both conjugated and isolated systems.