#799369: document possible sys.platform values.
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 6b88d61..c6fe2fa 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -507,9 +507,26 @@
.. data:: platform
- This string contains a platform identifier, e.g. ``'sunos5'`` or ``'linux1'``.
- This can be used to append platform-specific components to ``path``, for
- instance.
+ This string contains a platform identifier that can be used to append
+ platform-specific components to :data:`sys.path`, for instance.
+
+ For Unix systems, this is the lowercased OS name as returned by ``uname -s``
+ with the first part of the version as returned by ``uname -r`` appended,
+ e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
+ For other systems, the values are:
+
+ ================ ===========================
+ System :data:`platform` value
+ ================ ===========================
+ Windows ``'win32'``
+ Windows/Cygwin ``'cygwin'``
+ MacOS X ``'darwin'``
+ MacOS 9 ``'mac'``
+ OS/2 ``'os2'``
+ OS/2 EMX ``'os2emx'``
+ RiscOS ``'riscos'``
+ AtheOS ``'atheos'``
+ ================ ===========================
.. data:: prefix