In organic chemistry, hydrocarbons (compounds composed of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (/ˌælᵻˈfætᵻk/; G. aleiphar, fat, oil) also known as non-aromatic compounds. Aliphatics can be cyclic, but only aromatic compounds contain an especially stable ring of atoms, such as benzene. Aliphatic compounds can be saturated, like hexane, or unsaturated, like hexene. Open-chain compounds (whether straight or branched) contain no rings of any type, and are thus aliphatic.