In this region, I relied heavily on a single source from which I drew my list of places to include in the database as well as the countries and subdivisions in which they were located. The source, Erläuterungen zum Geschichtlicher Atlas der Rheinprovinz, is meant to accompany a map showing, as of 1789, the internal and external boundaries of the territory that became that Prussian province after the Napoleonic Wars. The text includes descriptions of boundary changes beginning in 1600 (of which there were remarkably few, making the need for many additional sources unnecessary). The map itself also has an insert covering the period between 1803 and 1806. The text and map can be found at the following links:
- Erläuterungen zum Geschichtlicher Atlas der Rheinprovinz – Google Books (text)
- Fabricius-Übersichtskarte_der_Rheinprovinz_von_1789.jpg (3618×5599) (wikimedia.org) (map)
Otherwise, German Wikipedia was sufficient to fill in the gaps.
Master list of countries and subdivisions in the Cologne region