diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
new file mode 100644
index 4a0e35a..6d39ce7
*** a/doc/src/sgml/runtime.sgml
--- b/doc/src/sgml/runtime.sgml
*************** postgres$ <userinput>initdb -D /usr/loca
*** 183,188 ****
--- 183,197 ----
     locale setting.  For details see <xref linkend="multibyte">.
    </para>
  
+   <para>
+    Non<literal>C</> and and non-<literal>POSIX</> locales rely on the
+    operating system's collation library for character set ordering.
+    This controls the ordering of keys stored in indexes.  For this reason,
+    a cluster cannot switch to an incompatible collation library version,
+    either through snapshot restore, binary streaming replication, or
+    <application>pg_upgrade</> run.
+   </para>
+ 
    <sect2 id="creating-cluster-mount-points">
     <title>Use of Secondary File Systems</title>