Azo compounds are compounds bearing the functional group diazenyl R−N=N−R′, in which R and R′ can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene (diimide), HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene." The more stable derivatives contain two aryl groups. The N=N group is called an azo group. The name azo comes from azote, the French name for nitrogen that is derived from the Greek ἀ- (a-, "not") + ζωή (zōē, life). Many textile and leather articles are dyed with azo dyes and pigments.