diff --git a/openslides/main.py b/openslides/main.py
index e4cd32b61..0d609b641 100644
--- a/openslides/main.py
+++ b/openslides/main.py
@@ -349,6 +349,14 @@ def get_user_data_path(*args):
 
 
 def is_portable():
+    """Return True if openslides is run as portable version"""
+
+    # NOTE: sys.executable is the path of the *interpreter*
+    #       the portable version embeds python so it *is* the interpreter.
+    #       The wrappers generated by pip and co. will spawn
+    #       the usual python(w).exe, so there is no danger of mistaking
+    #       them for the portable even though they may also be called
+    #       openslides.exe
     exename = os.path.basename(sys.executable).lower()
     return exename == "openslides.exe"