Many modern systematists, such as advocates of phylogenetic nomenclature, use cladistic methods that require taxa to be monophyletic (all descendants of some ancestor). Their basic unit, therefore, is the clade rather than the taxon. Similarly, among those contemporary taxonomists working with the traditional Linnean (binomial) nomenclature, few propose taxa they know to be paraphyletic. An example of a well-established taxon that is not also a clade is the class Reptilia, the reptiles; birds are descendants of reptiles but are not included in the Reptilia (birds are included in the Aves).