The theme of Septimius Severus, second founder of Byzantium and builder of edifices in this city, after the legendary Byzas and before Constantine, was spread through literary works written in Constantinople and in Antioch from the 6th century onwards. And yet, that is not confirmed by archaeology, and neither is it voiced in sources from the beginning of the 3rd century. I proposed therefore to broach this subject from a different perspective, by comparing the elements of this singular narrative with contemporary, historiographical and numismatic sources, and with the information on these geopolitical balances of an empire engaged in profound restructuring in the 3rd century. In comparison, the Byzantine corpus upon which I led my investigation is made up of two parts: historiographers, chronographers or abbreviators on the one hand, the Patria, poetic descriptions of sites or public monuments on the other hand. The classification of this information thus shows how, on the model of Antioch in Syria, Byzantium was endowed a posteriori with a rank that was not rightfully hers in the 3rd century, from a teleological perspective that has little relation with historical reality. With this operation, Nicomedia, neighbour of Byzantium, whose role throughout the 3rd century led it to go as far as to grow, like Cologne, Milan or Antioch, so that it became an imperial capital under Diocletian, was symbolically dispossessed and cast into the shadows for reasons of constantinopolitan civic pride.