diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index 46d7e19..a98e4af 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -1154,6 +1154,20 @@ SELCT 1/0;
     (if successful) or ErrorResponse (if not).
    </para>
 
+   <note>
+     <para>
+       Before <productname>PostgreSQL</productname> protocol 3.0, it was necessary
+       for the application to explicitly send the two characters
+       <literal>\.</literal> as a final line to indicate to the server that it
+       had finished sending <command>COPY</command> data.  Programs
+       implementing <command>COPY</command> in protocol 3.0
+       including <productname>PostgreSQL</productname> need to check and
+       ignore
+       <literal>\.</literal> just before COPYDone message for backward
+       compatibility sake. This requirement may be removed in the future.
+     </para>
+   </note>
+
    <para>
     In the event of a backend-detected error during copy-in mode (including
     receipt of a CopyFail message), the backend will issue an ErrorResponse