Potestaat was the title of a governor of medieval Friesland. According to the legendary 8th-century Karelsprivilege, which is probably a later forgery, Charlemagne had first granted the title of potestaat to Magnus Forteman. He and most of his early successors were probably entirely fictional, invented later by pseudohistorians in order to argue in favour of the notion of Frisian freedom. The title potestaat does not appear in historical documents until 1470, and at most, the earliest potestaat avant la lettre can be dated to the 13th century.
In 1851, Wopke Eekhoff compiled the following list in Beknopte Geschiedenis van Friesland ("Brief History of Friesland"). However, most of the list is fictitious.[1]
Most of the early potestaats are completely legendary.
From here the evidence for their existence is considerably better supported:
In 1464, Ulrich I of East Frisia was raised to the status of Count by Frederick III, Holy Roman Emperor, and East Frisia became a separate county. Continues in List of counts of East Frisia
With the victory of the Schieringers against the Vetkopers, the office passed to the dukes of Saxony: