The viceroys, who governed the other provinces, bore the title of Vitaxae, and were fourteen or fifteen in number.

The remark has been made by the historian Gibbon that the system thus established "exhibited under other names a lively image of the feudal system which has since prevailed in Europe." The comparison is of some value, but, like most historical parallels, it is inexact, the points of difference between the Parthian and the feudal system being probably more numerous than those of resemblance, but the points of resemblance being very main points, not fewer in number, and striking.
It was with special reference to the system thus established that the Parthian monarchs took the title of "King of Kings", so frequent upon their coins, which seems sometimes to have been exchanged for what was regarded as an equivalent phrase, "Satrap of Satraps".

This title seems to appear first on the coins of Mithridates I.
In the Parthian system there was one anomaly of a very curious character.

The Greek towns, which were scattered in large numbers throughout the Empire, enjoyed a municipal government of their own, and in some cases were almost independent communities, the Parthian kings exercising over them little or no control.

The great city of Seleucia on the Tigris was the most important of all these: its population was estimated in the first century after Christ at six hundred thousand souls; it had strong walls, and was surrounded by a most fertile territory.