Correct dermatome identification is essential to reduce nomenclature and surgical errors, to improve neurologic correlation, and to produce accurate statistics regarding low-back problems. The phylogenetic category of the spine must be known and junctional transitional vertebrae within the column properly identified and localized. This article emphasizes the appearances of, and the different positions of, transitional vertebrae at the thoracolumbar and lumbosacral junctions in the vertebral sequence depending on the phylogenetic category of the patient. It describes three additional characteristics of these anomalous vertebrae. Suggestions are made to eliminate the confusion and mistakes produced by the presence of these frequent variants.