diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 6646820d6a..a693424a69 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -3369,7 +3369,7 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
        </row>
        <row>
         <entry><type>line</type></entry>
-        <entry>32 bytes</entry>
+        <entry>24 bytes</entry>
         <entry>Infinite line</entry>
         <entry>{A,B,C}</entry>
        </row>
@@ -3413,6 +3413,11 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
      </tgroup>
     </table>
 
+   <para>
+    In all these types, the individual coordinates are stored as
+    <type>double precision</type> (<type>float8</type>) numbers.
+   </para>
+
    <para>
     A rich set of functions and operators is available to perform various geometric
     operations such as scaling, translation, rotation, and determining
@@ -3607,6 +3612,14 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
      within it, while a path is not.
     </para>
 
+    <para>
+     An important implementation difference between polygons and closed
+     paths is that the stored representation of a polygon includes its
+     smallest bounding box.  This speeds up certain search operations,
+     although computing the bounding box adds overhead while constructing
+     new polygons.
+    </para>
+
     <para>
      Values of type <type>polygon</type> are specified using any of the
      following syntaxes: