range type, as described in <xref linkend="rangetypes"/>.
   </para>

   <para>
    The range type's <replaceable class="parameter">subtype</replaceable> can
    be any type with an associated b-tree operator class (to determine the
    ordering of values for the range type).  Normally the subtype's default
    b-tree operator class is used to determine ordering; to use a non-default
    operator class, specify its name with <replaceable
    class="parameter">subtype_opclass</replaceable>.  If the subtype is
    collatable, and you want to use a non-default collation in the range's
    ordering, specify the desired collation with the <replaceable
    class="parameter">collation</replaceable> option.
   </para>

   <para>
    The optional <replaceable class="parameter">canonical</replaceable>
    function must take one argument of the range type being defined, and
    return a value of the same type.  This is used to convert range values
    to a canonical form, when applicable.  See <xref
    linkend="rangetypes-defining"/> for more information.  Creating a
    <replaceable class="parameter">canonical</replaceable> function
    is a bit tricky, since it must be defined before the range type can be
    declared.  To do this, you must first create a shell type, which is a
    placeholder type that has no properties except a name and an
    owner.  This is done by issuing the command <literal>CREATE TYPE
    <replaceable>name</replaceable></literal>, with no additional parameters.  Then
    the function can be declared using the shell type as argument and result,
    and finally the range type can be declared using the same name.  This
    automatically replaces the shell type entry with a valid range type.
   </para>

   <para>
    The optional <replaceable class="parameter">subtype_diff</replaceable>
    function must take two values of the
    <replaceable class="parameter">subtype</replaceable> type as argument,
    and return a <type>double precision</type> value representing the
    difference between the two given values.  While this is optional,
    providing it allows much greater efficiency of GiST indexes on columns of
    the range type.  See <xref linkend="rangetypes-defining"/> for more
    information.
   </para>

   <para>
    The optional <replaceable class="parameter">multirange_type_name</replaceable>
    parameter specifies the name of the corresponding multirange type.  If not
    specified, this name is chosen automatically as follows.
    If the range type name contains the substring <literal>range</literal>, then
    the multirange type name is formed by replacement of the <literal>range</literal>
    substring with <literal>multirange</literal> in the range
    type name.  Otherwise, the multirange type name is formed by appending a
    <literal>_multirange</literal> suffix to the range type name.
   </para>
  </refsect2>

  <refsect2>
   <title>Base Types</title>

  <para>
   The fourth form of <command>CREATE TYPE</command> creates a new base type
   (scalar type).  To create a new base type, you must be a superuser.
   (This restriction is made because an erroneous type definition could
   confuse or even crash the server.)
  </para>

  <para>
   The parameters can appear in any order, not only that
   illustrated above, and most are optional.  You must register
   two or more functions (using <command>CREATE FUNCTION</command>) before
   defining the type.  The support functions
   <replaceable class="parameter">input_function</replaceable> and
   <replaceable class="parameter">output_function</replaceable>
   are required, while the functions
   <replaceable class="parameter">receive_function</replaceable>,
   <replaceable class="parameter">send_function</replaceable>,
   <replaceable class="parameter">type_modifier_input_function</replaceable>,
   <replaceable class="parameter">type_modifier_output_function</replaceable>,
   <replaceable class="parameter">analyze_function</replaceable>, and
   <replaceable