The power which possessed Parthia, Hyrcania, and Bactria, would rule almost of necessity over the whole tract between the Elburz range and the Oxus, if not even over the region between the Oxus and the Jaxartes; that which held the Caspian mountains and eastern Media could not fail to have influence over the tribes of the Iranic desert; while Assyria Proper would naturally follow the fortunes of Babylonia and Susiana.

Still the extent of territory thus indicated rests only on conjecture.

If we confine ourselves to what is known by positive evidence, we can only say that the Parthian Kingdom of this period contained, at least, twelve provinces above enumerated.

It thus stretched from east to west a distance of fifteen hundred miles between the Suleiman mountains and the Euphrates, varying in width from three or four hundred miles--or even more--towards the west and east, to a narrow strip of less than a hundred miles toward the centre.

It probably comprised an area of about 450,000 square miles; which is somewhat less than that of the modern Persia.
Unlike the modern Persia, however, the territory consisted almost entirely of productive regions.