This tract, which was contiguous to Arzanene and Zabdicene, if we have rightly placed those regions, must almost certainly have been the Cordyene of the treaty, which, if it corresponded at all nearly in extent with the modern Kurdistan, must have been by far the largest and most important of the five provinces.
The two remaining tracts, whatever their names, must undoubtedly have lain on the same side of the Tigris with these three.

As they are otherwise unknown to us (for Sophene, which had long been Roman, cannot have been one of them), it is impossible that they should have been of much importance.

No doubt they helped to round off the Roman dominion in this quarter; but the great value of the entire cession lay in the acquisition of the large and fruitful province of Cordyene, inhabited by a brave and hardy population, and afterwards the seat of fifteen fortresses which brought the Roman dominion to the very edge of Adiabene, made them masters of the passes into Media, and laid the whole of Southern Mesopotamia open to their incursions.

It is probable that the hold of Persia on the territory had never been strong; and in relinquishing it she may have imagined that she gave up no very great advantage; but in the hands of Rome Kurdistan became a standing menace to the Persian power, and we shall find that on the first opportunity the false step now taken was retrieved, Cordyene with its adjoining districts was pertinaciously demanded of the Romans, was grudgingly surrendered, and was then firmly re-attached to the Sassanian dominions.
(ii.) The Tigris is said by Patricius and Festus to have been made the boundary of the two empires.

Gibbon here boldly substitutes the Western Khabour and maintains that "the Roman frontier traversed, but never followed, the course of the Tigris." He appears not to be able to understand how the Tigris could be the frontier, when five provinces across the Tigris were Roman.