For some authors, only states that have a border with the US are considered as northern Mexico, but this description would exclude Durango, Sinaloa and Baja California Sur.
For some authors, only states that have a border with the US are considered as northern Mexico, i.e. Baja California, Chihuahua, Coahuila, Nuevo León, Sonora and Tamaulipas. Others also include Durango, Sinaloa and Baja California Sur.