In biology, a genus (plural: genera) is a taxonomic rank used in the biological classification of living and fossil organisms. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus. e.g., Pongo pygmaeus and Pongo abelii are two species within the genus Pongo. Pongo is a genus within the family Hominidae. The composition of a genus is determined by a taxonomist. The standards for genus classification are not strictly codified, so different authorities often produce different classifications for genera. There are some general practices used, however, including the idea that a newly defined genus should fulfill these three criteria to be descriptively useful: monophyly – all descendants of an ancestral taxon are grouped together; reasonable compactness – a genus should not be expanded needlessly; and distinctness – with respect to evolutionarily relevant criteria, i.e. ecology, morphology, or biogeography; note that DNA sequences are a consequence rather than a condition of diverging evolutionary lineages except in cases where they directly inhibit gene flow (e.g. postzygotic barriers). The term comes from the Latin genus meaning “descent, family, type, gender”, cognate with – genos, “race, stock, kin”.