diff --git a/README.md b/README.md
index 71b5d968b..985385a86 100644
--- a/README.md
+++ b/README.md
@@ -59,9 +59,14 @@ Download OpenStreetMap extracts for example from [Geofabrik](http://download.geo
 Pre-process the extract with the car profile and start a routing engine HTTP server on port 5000
 
     docker run -t -v $(pwd):/data osrm/osrm-backend osrm-extract -p /opt/car.lua /data/berlin-latest.osm.pbf
+
+The flag `-v $(pwd):/data` creates the directory `/data` inside the docker container, and makes the current working directory `$(pwd)` available there. The file `/data/berlin-latest.osm.pbf` inside the container is referring to `berlin-latest.osm.pbf` on the host.
+
     docker run -t -v $(pwd):/data osrm/osrm-backend osrm-partition /data/berlin-latest.osrm
     docker run -t -v $(pwd):/data osrm/osrm-backend osrm-customize /data/berlin-latest.osrm
 
+Note that `berlin-latest.osrm` has a different file extension. 
+
     docker run -t -i -p 5000:5000 -v $(pwd):/data osrm/osrm-backend osrm-routed --algorithm mld /data/berlin-latest.osrm
 
 Make requests against the HTTP server