The senior ecclesiastical principality of the Holy Roman Empire — its archbishop crowned emperors and chaired the electoral college for a thousand years.
Secularized in the 1803 mediatization; its scattered lands went to Hesse and other neighbours.
The atlas holds 4 border frames for Electorate of Mainz — the shape you scrub through changes in 780 · 915 · 1269 · 1749.
The imperial church and electoral system Mainz anchored.