Dinosaurs have two-part scientific names (such as Tyrannosaurus rex) applied according to a system first created by Carl Linnaeus in 1735. The first name, or genus name (Tyrannosaurus in this example), always begins with a capital letter. The second name, or species name (rex), always begins with a small letter (lower case letter) and technically denotes a variety of that genus. Hence, the species name should never appear alone. It should always be coupled with the genus. Thus, "Tyrannosaurus rex" or "T. rex" (abbreviating the genus name) are proper, but "rex" all by itself would not be considered correct.
By convention, the genus and species names are always placed in italics or underlined. This web site uses italics, which is the preferred formatting. The name is usually derived from Greek and/or Latin - the classical languages known to all well-educated people prior to the 20th Century. Hence, Tyrannosaurus ("Tyrant Lizard") is a combination of Latin (Tyrannus) + Greek (Sauros), and rex ("king") is Latin. The first scientist to describe a new genus or species in a formal publication has the honor of naming it. This scientist will designate a single specimen, known as a Type Specimen, to be the name holder for the new dinosaur. In referring additional specimens to this same dinosaur, scientists must always compare their fossil material to the original Type specimen. In the case of Tyrannosaurus, the type is a skeleton on display at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. It should be noted that the Type does not have to be an entire skeleton - sometimes it is nothing more than a single bone!
There are mutually agreed upon international rules that govern the use of scientific names. One such rule says that, if two names are found to represent the same creature, then the older name takes precedence. Hence, when it was discovered that Brontosaurus and Apatosaurus were actually the very same dinosaur, the name Brontosaurus became invalid and the correct name was recognized as Apatosaurus (which had been published first). However, Brontosaurus was (and remains) a much more popular name with the public.
If two names representing the same dinosaur should appear in the very same publication, then the first scientist to sort out the mess is allowed to say which name should be officially used. It will most often be the one whose Type Specimen is the most complete or has the most identifiable features. Years ago, however, the name that was chosen was often the one that appeared first in the publication (known as page priority), but this is no longer recognized as a valid method.
The formal rules that govern the naming of dinosaurs are laid down in the International Code of Zoological Nomenclature - a mouthful of large words that simply means "the international rules for naming animals" (dinosaurs are members of the Animal Kingdom and Zoology is the scientific study of animals).
The international rules exist to help stabilize the use of scientific names, so that scientists all over the world can communicate without needless confusion.