Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons) are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings (organic rings in which the electrons are delocalized). Formally, the class is further defined as lacking further branching substituents off of these ring structures. Polynuclear aromatic hydrocarbons (PNAs) are a subset of PAHs that have fused aromatic rings, that is, rings that share one or more sides. Though poly- in these cases literally means "many", there is precedence in nomenclature for beginning this class and subclass with the two ring cases, where  naphthalene would therefore be considered a simple example; beginning at three rings, examples include anthracene and phenanthrene.