Postal-code data: GeoNames, CC BY 4.0. Source: https://download.geonames.org/export/zip/DE.zip
https://www.geonames.org/about.html
https://creativecommons.org/licenses/by/4.0/
Retrieved 2026-09-16. German subset filtered to records with accuracy 4 or 6, deduplicated and converted for approximate postal-code distance matching in Helfmann-Portal. No warranty of accuracy.