An aromatic hydrocarbon, or arene, is a hydrocarbon ring compound. It has alternating double and single bonds between carbon atoms forming rings. Many of the compounds have a sweet scent, hence the term 'aromatic'. The ring of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible hydrocarbon ring, benzene. Aromatic hydrocarbons can be "monocyclic" (MAH) or "polycyclic" (PAH).