diff --git a/runtime/doc/vi_diff.txt b/runtime/doc/vi_diff.txt
index 82f6274..f762e8a 100644
--- a/runtime/doc/vi_diff.txt
+++ b/runtime/doc/vi_diff.txt
@@ -1,4 +1,4 @@
-*vi_diff.txt*   For Vim version 8.1.  Last change: 2019 Oct 12
+*vi_diff.txt*   For Vim version 8.1.  Last change: 2019 Nov 12
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -1323,6 +1323,11 @@
 flags when Vim starts up.  This makes Vim run as POSIX as it can.  That's
 a bit different from being Vi compatible.
 
+You can find the Posix specification for Vi here:
+https://pubs.opengroup.org/onlinepubs/9699919799/utilities/vi.html
+And the related Ex specification:
+https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ex.html
+
 This is where Vim does not behave as POSIX specifies and why:
 
 							*posix-screen-size*