#5276: document IDLESTARTUP and .Idle.py.
diff --git a/Doc/library/idle.rst b/Doc/library/idle.rst
index 413750f..1b78fb9 100644
--- a/Doc/library/idle.rst
+++ b/Doc/library/idle.rst
@@ -253,6 +253,24 @@
black
+Startup
+-------
+
+Upon startup with the ``-s`` option, IDLE will execute the file referenced by
+the environment variables :envvar:`IDLESTARTUP` or :envvar:`PYTHONSTARTUP`.
+Idle first checks for ``IDLESTARTUP``; if ``IDLESTARTUP`` is present the file
+referenced is run. If ``IDLESTARTUP`` is not present, Idle checks for
+``PYTHONSTARTUP``. Files referenced by these environment variables are
+convenient places to store functions that are used frequently from the Idle
+shell, or for executing import statements to import common modules.
+
+In addition, ``Tk`` also loads a startup file if it is present. Note that the
+Tk file is loaded unconditionally. This additional file is ``.Idle.py`` and is
+looked for in the user's home directory. Statements in this file will be
+executed in the Tk namespace, so this file is not useful for importing functions
+to be used from Idle's Python shell.
+
+
Command line usage
^^^^^^^^^^^^^^^^^^