This essay deals with a classical theme in ancient history: the relationships between Rome and the Parthian and Sasanian worlds. What is original is the perspective from which these relationships are investigated, i.e. starting from the analysis of titles apparently used in a non-fitting context on both sides of the border. A very Roman concept, the consularitas, Greek hypateia, is analysed on the one hand in its explicit attribution to the Edessean sovereigns, on the other hand in its implicit attribution to the kings of Armenia and afterwards to the 'Chiefs' of Palmyra. The very Iranian title of 'King of Kings' is also investigated here as unsuitably attributed to Odainath of Palmyra and his son Vaballath, together with an analysis of the very Iranian title argabetes of a very powerful Palmyrenean man, Vorod. This essay is completed by a documentary update on another Iranian title, pasgriba, 'heir to the throne, crownprince', attested in the royal families of Edessa and Hatra, whose meaning has been deeply investigated elsewhere. The above mentioned subjects evidently show how tight the bonds linking the two sides of the river Euphrates used to be, bonds that are not only cultural in a general way, but more concretely also institutional and political.