diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 8d7d9a2f3e8..a6d79765c1a 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -1824,13 +1824,23 @@ SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in
         which <parameter>operand</parameter> falls in a histogram
         having <parameter>count</parameter> equal-width buckets spanning the
         range <parameter>low</parameter> to <parameter>high</parameter>.
-        Returns <literal>0</literal>
+        The buckets have inclusive lower bounds and exclusive upper bounds.
+        Returns <literal>0</literal> for an input less
+        than <parameter>low</parameter>,
         or <literal><parameter>count</parameter>+1</literal> for an input
-        outside that range.
+        greater than or equal to <parameter>high</parameter>.
+        If <parameter>low</parameter> &gt; <parameter>high</parameter>,
+        the behavior is mirror-reversed, with bucket <literal>1</literal>
+        now being the one just below <parameter>low</parameter>, and the
+        inclusive bounds now being on the upper side.
        </para>
        <para>
         <literal>width_bucket(5.35, 0.024, 10.06, 5)</literal>
         <returnvalue>3</returnvalue>
+       </para>
+       <para>
+        <literal>width_bucket(9, 10, 0, 10)</literal>
+        <returnvalue>2</returnvalue>
        </para></entry>
       </row>
 
@@ -1842,8 +1852,8 @@ SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in
        <para>
         Returns the number of the bucket in
         which <parameter>operand</parameter> falls given an array listing the
-        lower bounds of the buckets.  Returns <literal>0</literal> for an
-        input less than the first lower
+        inclusive lower bounds of the buckets.
+        Returns <literal>0</literal> for an input less than the first lower
         bound.  <parameter>operand</parameter> and the array elements can be
         of any type having standard comparison operators.
         The <parameter>thresholds</parameter> array <emphasis>must be
diff --git a/src/backend/utils/adt/float.c b/src/backend/utils/adt/float.c
index 6d20ae07ae7..ba66a9c4ce6 100644
--- a/src/backend/utils/adt/float.c
+++ b/src/backend/utils/adt/float.c
@@ -4065,8 +4065,8 @@ float84ge(PG_FUNCTION_ARGS)
  * in the histogram. width_bucket() returns an integer indicating the
  * bucket number that 'operand' belongs to in an equiwidth histogram
  * with the specified characteristics. An operand smaller than the
- * lower bound is assigned to bucket 0. An operand greater than the
- * upper bound is assigned to an additional bucket (with number
+ * lower bound is assigned to bucket 0. An operand greater than or equal
+ * to the upper bound is assigned to an additional bucket (with number
  * count+1). We don't allow "NaN" for any of the float8 inputs, and we
  * don't allow either of the histogram bounds to be +/- infinity.
  */
diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c
index 40dcbc7b671..58ad1a65ef7 100644
--- a/src/backend/utils/adt/numeric.c
+++ b/src/backend/utils/adt/numeric.c
@@ -1958,9 +1958,10 @@ generate_series_numeric_support(PG_FUNCTION_ARGS)
  * in the histogram. width_bucket() returns an integer indicating the
  * bucket number that 'operand' belongs to in an equiwidth histogram
  * with the specified characteristics. An operand smaller than the
- * lower bound is assigned to bucket 0. An operand greater than the
- * upper bound is assigned to an additional bucket (with number
- * count+1). We don't allow "NaN" for any of the numeric arguments.
+ * lower bound is assigned to bucket 0. An operand greater than or equal
+ * to the upper bound is assigned to an additional bucket (with number
+ * count+1). We don't allow "NaN" for any of the numeric inputs, and we
+ * don't allow either of the histogram bounds to be +/- infinity.
  */
 Datum
 width_bucket_numeric(PG_FUNCTION_ARGS)