Giving any living or extinct organism a scientific name is governed by a set of rules called the International Code of Zoological Nomenclature, and paleontologists who name dinosaurs must follow those rules to make the name valid. The code requires that a scientific name be composed to two parts. The first part, called the genus, is always capitalized; the second, called the species is never capitalized. Both names are always italicized, and sometimes the genus name is abbreviated (as in T. rex for Tyrannosaurus rex). The genus name may be used alone to refer to all the species in a particular genus.