Also known as al-Madāʾin. Located on the Tigris, south of modern Baghdad. The original Seleucia on the Tigris was established on the west side of the river at the end of the 4th cent. BC as a Hellenistic center by Seleucus I Nicator. Seleucia was largely abandoned for a new center at Ctesiphon during the Parthian period, possibly due to a shift in the course of the Tigris. Although some occupation remained in this area, excavations by an American team indicate that the occupation was primarily industrial. Although the Parthians established Ctesiphon as their capital, its exact location remains uncertain. Based on archaeological material, most scholars locate it on the east side of the Tigris, north of Kokhe. Sasanian and Syriac references to Seleucia-Ctesiphon almost certainly refer not to Old Seleucia, but to the city established by the Sasanians on the right bank of the Tigris as their administrative capital, which was known as New Seleucia, Kokhe, or Veh-Ardashir. Seleucia-Ctesiphon was home to the Cath. of the Ch. of E. from at least the early 5th cent. This is known from the Synodicon Orientale, which indicates that the synod of Isḥaq, the first synod, was held in the see of Seleucia-Ctesiphon in 410.