diff --git a/docs/narr.rst b/docs/narr.rst
index b34f928..0bcc435 100644
--- a/docs/narr.rst
+++ b/docs/narr.rst
@@ -178,7 +178,8 @@ The following options are only needed one time as their values are stored in
 
 --oldest-python
   The oldest version of Python supported by this package. Specified as version
-  number, e.g. ``3.8``.
+  number, e.g. ``3.8``. This setting is optional and defaults to the lowest
+  Python version generally supported by zopefoundation packages.
 
 --with-docs
   Enable building the documentation using Sphinx. This will also create a