*** a/doc/src/sgml/datatype.sgml
--- b/doc/src/sgml/datatype.sgml
***************
*** 1622,1627 **** HOUR TO MINUTE
--- 1622,1630 ----
  HOUR TO SECOND
  MINUTE TO SECOND
  </literallayout>
+     These only zero less-significant fields, making <literal>DAY TO
+     MINUTE</>, <literal>HOUR TO MINUTE</> and <literal>MINUTE</> equivalent.
+     The redundant forms exist for compatibility with the SQL standard.
      Note that if both <replaceable>fields</replaceable> and
      <replaceable>p</replaceable> are specified, the
      <replaceable>fields</replaceable> must include <literal>SECOND</>,