diff --git a/app.py b/app.py
index 585091e..1cf27d8 100755
--- a/app.py
+++ b/app.py
@@ -58,6 +58,10 @@ GEOCODE_REGIONS = ["Baden-Württemberg", "Hessen"]
 MAP_DEFAULT_CENTER = [49.0, 9.0]
 MAP_DEFAULT_ZOOM = 8
 
+#: Kartenkacheln. "OpenStreetMap" braucht keinen API-Schlüssel.
+#: (Die früher genutzten "CartoDB positron"-Kacheln verlangen seit 2024 einen Schlüssel.)
+MAP_TILES = "OpenStreetMap"
+
 log = logging.getLogger("arbeitsorte")
 
 
@@ -415,7 +419,7 @@ def generate_map(store: DataStore) -> Path:
 
     m = folium.Map(
         location=MAP_DEFAULT_CENTER, zoom_start=MAP_DEFAULT_ZOOM,
-        tiles="CartoDB positron",
+        tiles=MAP_TILES,
     )
     max_count = int(freq["count"].max())
 
diff --git a/docs/architecture.md b/docs/architecture.md
index fffd22c..45233d7 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -84,6 +84,6 @@ This is the file to ask for when diagnosing a problem on the user's laptop.
 |------------|-----------|-------------------|
 | Nominatim (OSM) | geocoding new/edited entries | entries save with blank coords, shown as `⚠ nicht gefunden` |
 | CDN (jsdelivr, jquery, cloudflare) | rendering `karte.html` | map page loads blank / unstyled |
-| CartoDB tiles | map background | grey tiles |
+| OpenStreetMap tiles (`tile.openstreetmap.org`) | map background | blank/grey tiles |
 
 Existing entries that already have coordinates do **not** need the network.
diff --git a/docs/changelog.md b/docs/changelog.md
index a5e12eb..ebfa79d 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -2,6 +2,14 @@
 
 Notable changes to the app. Newest first.
 
+## 2026-09-07 — map tiles switched to OpenStreetMap
+
+`folium.Map(tiles="CartoDB positron")` began showing an "API KEY REQUIRED"
+watermark (Carto moved its basemaps behind a key). Switched to
+`tiles="OpenStreetMap"` via a new `MAP_TILES` constant — no key required. OSM
+standard tiles are more colourful than positron was; `MAP_TILES` makes swapping
+the provider a one-line change.
+
 ## 2026-09-07 — UI/UX pass
 
 A round of usability improvements aimed at the non-technical user
diff --git a/docs/dev-notes.md b/docs/dev-notes.md
index 796722a..1e138fb 100644
--- a/docs/dev-notes.md
+++ b/docs/dev-notes.md
@@ -36,6 +36,20 @@ Numeric parsing is deliberately deferred to `get_map_data` (`pd.to_numeric(...,
 errors="coerce")`). Don't "fix" columns to float on load — blank coordinates and
 leading-zero PLZ both depend on the string representation.
 
+## CartoDB "positron" tiles now require an API key (2026-09-07)
+
+The original `folium.Map(tiles="CartoDB positron")` started rendering an
+"API KEY REQUIRED / carto.com/basemaps/apikey" watermark – Carto moved its
+basemaps behind a key. Switched to `tiles="OpenStreetMap"` (constant
+`MAP_TILES`), which needs no key. Trade-off: OSM standard tiles are more
+colourful than positron's muted grey. Other no-key options if a lighter look is
+wanted: OSM Germany (`tile.openstreetmap.de`), OSM France Hot
+(`{s}.tile.openstreetmap.fr/hot`) – both still colourful. Truly muted
+key-free basemaps are hard to find now.
+
+Don't assume a "free" third-party tile source stays free – pin the provider in
+one constant so swapping it is a one-line change.
+
 ## Nominatim query order matters
 
 `geocode_city` returns the **first** hit, trying PLZ-qualified first, then each
diff --git a/docs/improvements.md b/docs/improvements.md
index aefc02b..075b384 100644
--- a/docs/improvements.md
+++ b/docs/improvements.md
@@ -168,11 +168,12 @@ or at least skip exact duplicates by default.
 
 ## 9. Offline map 🟢
 
-`karte.html` loads Leaflet/jQuery/Bootstrap from CDNs and tiles from CartoDB, so
-the map is blank without internet even though all the data is local. If offline
-use matters, vendor the Leaflet JS/CSS into `data/` and post-process the Folium
-output to point at local files (tiles would still need caching or an offline
-tile pack — larger effort).
+`karte.html` loads Leaflet/jQuery/Bootstrap from CDNs and tiles from
+`tile.openstreetmap.org` (`MAP_TILES`; switched away from CartoDB positron, which
+now needs an API key). The map is blank without internet even though all the
+data is local. If offline use matters, vendor the Leaflet JS/CSS into `data/`
+and post-process the Folium output to point at local files (tiles would still
+need caching or an offline tile pack — larger effort).
 
 ## 10. Small code-quality items 🟢 — partly done
 
diff --git a/docs/overview.md b/docs/overview.md
index f742748..3bc828e 100644
--- a/docs/overview.md
+++ b/docs/overview.md
@@ -62,8 +62,9 @@ A table view of everything in `orte.csv`.
 
 Generated by `generate_map()`:
 
-- Base layer: `CartoDB positron`, initial view centred on `[49.0, 9.0]`,
-  zoom 8 (roughly Baden-Württemberg).
+- Base layer: **OpenStreetMap** standard tiles (`MAP_TILES`) – no API key.
+  Initial view centred on `[49.0, 9.0]`, zoom 8 (roughly Baden-Württemberg);
+  the view auto-fits to the markers when there is more than one.
 - One **`CircleMarker`** per unique `(city, lat, lon)` group. Radius scales
   linearly with the visit count for that location
   (`5 + count / max_count * 20` px), colour is DRK red (`#CC0000`).
@@ -72,8 +73,12 @@ Generated by `generate_map()`:
 - If there are no usable coordinates at all, a dialog says so (and the status
   bar shows a warning).
 
-> The generated HTML pulls Leaflet, jQuery and Bootstrap from CDNs, so the map
-> needs an internet connection to render even though the data is local.
+> The generated HTML pulls Leaflet, jQuery and Bootstrap from CDNs, and the map
+> tiles from `tile.openstreetmap.org`, so the map needs an internet connection to
+> render even though the data is local. To change the look, set `MAP_TILES` near
+> the top of `app.py` to another no-key provider Folium knows
+> (`"OpenStreetMap"`, `"CartoDB positron"` *now needs a key*, or a custom
+> `folium.TileLayer` URL such as OSM Germany / OSM France).
 
 ## Geocoding behaviour