The name Frankfurt first appears in writing in the year 793, but it seems to have already been a considerable city. In 794 a letter from the Emperor to the bishop of Toledo contained "in loco celebri, qui dicitur Franconofurd", which reads "that famous place, which is called Frankfurt."
It seems Cathedral Hill was already permanently settled in Merovingian times (possibly first by Romans). In 1992 excavations at the cathedral found the rich grave of a girl, that has been dated to the late Merovingian period of the 7th century.
Charlemagne built himself a royal court at "Franconovurd", the "ford of the Franks", and in the summer of 794 held a church council there, convened by the grace of God, authority of the pope, and command of Charlemagne (canon 1), and attended by the bishops of the Frankish kingdom, Italy and the province of Aquitania, and even by ecclesiastics from England. The council was summoned primarily for the condemnation of Adoptionism. According to the testimony of contemporaries two papal legates were present, Theophylact and Stephen, representing Pope Adrian I. After an allocution by Charlemagne, the bishops drew up two memorials against the Adoptionists, one containing arguments from patristic writings; the other arguments from Scripture. The first was the Libellus sacrosyllabus, written by Paulinus, Patriarch of Aquileia, in the name of the Italian bishops; the second was the Epistola Synodica, addressed to the bishops of Spain by those of Germany, Gaul and Aquitania. In the first of its fifty-six canons the council condemned Adoptionism, and in the second repudiated the Second Council of Nicaea of 787, which, according to the faulty Latin translation of its Acts (see Caroline Books), seemed to decree that the same kind of worship should be paid to images as to the Blessed Trinity, though the Greek text clearly distinguishes between latreia and proskynesis; this constituted a condemnation of iconoclasm. The remaining fifty-four canons dealt with metropolitan jurisdiction, monastic discipline, superstition etc.
Louis the Pious, Charlemagne's son, selected Frankfurt as his seat, extended the palatinate, built a larger palace, and in 838 had the city encircled by defensive walls and ditches.
After the Treaty of Verdun (843), Frankfurt became to all intents and purposes the capital of East Francia and was named Principalis sedes regni orientalis (principal seat of the eastern realm). Kings and emperors frequently stayed in Frankfurt, and Reichstags and church councils were repeatedly held there. The establishment of religious monasteries and numerous endowments to the local church furthered the urban community. Also, as the German emperor had no permanent residence anymore, Frankfurt remained the center of imperial power and the principal city of Eastern Francia.