diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
new file mode 100644
index 02763b5..f33f29d
*** a/doc/src/sgml/datatype.sgml
--- b/doc/src/sgml/datatype.sgml
*************** NUMERIC
*** 681,686 ****
--- 681,695 ----
       from zero will cause an underflow error.
      </para>
  
+     <para>
+      The (inherently inaccurate) textual representation of a <type>real</type>
+      or <type>double precision</type> value will by default contain only as many
+      significant digits as can be represented without losing precision
+      on any platform supported by PostgreSQL.  The configuration
+      parameter <xref linkend="guc-extra-float-digits"> can be used to
+      adjust this number.
+     </para>
+ 
      <indexterm>
       <primary>not a number</primary>
       <secondary>double precision</secondary>