St. John's Church is located beneath Mainz Cathedral in the historical center of Mainz, Germany. This 1100-year-old church was the first episcopal see of the Bishop of Mainz. It is the oldest church in Mainz, the oldest cathedral in the Germany of today and the only preserved cathedral building from late Carolingian and early Ottonian time in Germany.

Mainz Cathedral or St. Martin's Cathedral is located near the historical center and pedestrianized market square of the city of Mainz, Germany. This 1000-year-old Roman Catholic cathedral is the site of the episcopal see of the Bishop of Mainz.