Holstein-Plön

Holstein-Plön was a county in the Holy Roman Empire centred on Plön in Holstein. It was held by the House of Schauenburg who had ruled in Holstein and Stormarn since 1110/11. The county emerged before 1295 when the County of Holstein-Itzehoe was partitioned after the death of Count Gerhard I of Holstein-Itzehoe into the counties of Holstein-Plön, Holstein-Pinneberg and Holstein-Rendsburg.
The following counts ruled over Holstein-Plön:

1295–1312 Gerhard II the Blind, married Agnes of Brandenburg, daughter of John I of Brandenburg, in 1293
1312–1314 Gerhard IV, dean of Lübeck Cathedral, 1300–1311
1312–1359 John III the Mild
1323–1350 was never a ruler, but only a landowner and canon at Lübeck Cathedral
1321–1359 John III the Mild, also Count of Holstein-Kiel and Lord of Fehmarn
1359–1390 Adolphus VII, also Count of Holstein-Kiel and Lord of Fehmarn

Following the death of Gerhard II his sons, Gerhard IV of Holstein-Plön and his younger half-brother John III, ruled jointly over Holstein-Plön. In 1314 Gerhard IV sold his lands to John III, who thus became the sole Count of Holstein-Plön. John III conquered Holstein-Kiel with Gerhard III of Holstein-Rendsburg. Holstein-Plön reverted to Holstein-Kiel in 1350 upon the death of 27-year-old Gerhard V without issue.
When the Plön line died out in 1390, Holstein-Kiel reverted to Holstein-Rendsburg, ruled by Count Nicholas of Holstein-Rendsburg and his nephew, Count Gerhard VI of Holstein-Rendsburg. A circa-1400 map of the region shows the borders of Holstein-Rendsburg and its subordinate counties, Holstein-Kiel and Holstein-Plön.