diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml
index f6c4a4defa1..9e87186bb53 100644
--- a/doc/src/sgml/syntax.sgml
+++ b/doc/src/sgml/syntax.sgml
@@ -1709,10 +1709,10 @@ SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect
     The syntax of a window function call is one of the following:
 
 <synopsis>
-<replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) OVER ( <replaceable class="parameter">window_definition</replaceable> )
 <replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) OVER <replaceable>window_name</replaceable>
-<replaceable>function_name</replaceable> ( * ) OVER ( <replaceable class="parameter">window_definition</replaceable> )
+<replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) OVER ( <replaceable class="parameter">window_definition</replaceable> )
 <replaceable>function_name</replaceable> ( * ) OVER <replaceable>window_name</replaceable>
+<replaceable>function_name</replaceable> ( * ) OVER ( <replaceable class="parameter">window_definition</replaceable> )
 </synopsis>
     where <replaceable class="parameter">window_definition</replaceable>
     has the syntax
@@ -1749,15 +1749,14 @@ UNBOUNDED FOLLOWING
     names or numbers.
     <replaceable>window_name</replaceable> is a reference to a named window
     specification defined in the query's <literal>WINDOW</literal> clause.
-    Named window specifications are usually referenced with just
-    <literal>OVER</> <replaceable>window_name</replaceable>, but it is
-    also possible to write a window name inside the parentheses and then
-    optionally supply an ordering clause and/or frame clause (the referenced
-    window must lack these clauses, if they are supplied here).
-    This latter syntax follows the same rules as modifying an existing
-    window name within the <literal>WINDOW</literal> clause; see the
-    <xref linkend="sql-select"> reference
-    page for details.
+    Alternatively, a full <replaceable>window_definition</replaceable> can
+    be given within parentheses, using the same syntax as for defining a
+    named window in the <literal>WINDOW</literal> clause; see the
+    <xref linkend="sql-select"> reference page for details.  It's worth
+    pointing out that <literal>OVER wname</> is not exactly equivalent to
+    <literal>OVER (wname)</>; the latter implies copying and modifying the
+    window definition, and will be rejected if the referenced window
+    specification includes a frame clause.
    </para>
 
    <para>
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index c5fe6b6a3fd..8f697f85ec5 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -1665,11 +1665,16 @@ transformWindowDefinitions(ParseState *pstate,
 		/*
 		 * Per spec, a windowdef that references a previous one copies the
 		 * previous partition clause (and mustn't specify its own).  It can
-		 * specify its own ordering clause. but only if the previous one had
+		 * specify its own ordering clause, but only if the previous one had
 		 * none.  It always specifies its own frame clause, and the previous
-		 * one must not have a frame clause.  (Yeah, it's bizarre that each of
+		 * one must not have a frame clause.  Yeah, it's bizarre that each of
 		 * these cases works differently, but SQL:2008 says so; see 7.11
-		 * <window clause> syntax rule 10 and general rule 1.)
+		 * <window clause> syntax rule 10 and general rule 1.  The frame
+		 * clause rule is especially bizarre because it makes "OVER foo"
+		 * different from "OVER (foo)", and requires the latter to throw an
+		 * error if foo has a nondefault frame clause.	Well, ours not to
+		 * reason why, but we do go out of our way to throw a useful error
+		 * message for such cases.
 		 */
 		if (refwc)
 		{
@@ -1708,11 +1713,27 @@ transformWindowDefinitions(ParseState *pstate,
 			wc->copiedOrder = false;
 		}
 		if (refwc && refwc->frameOptions != FRAMEOPTION_DEFAULTS)
+		{
+			/*
+			 * Use this message if this is a WINDOW clause, or if it's an OVER
+			 * clause that includes ORDER BY or framing clauses.  (We already
+			 * rejected PARTITION BY above, so no need to check that.)
+			 */
+			if (windef->name ||
+				orderClause || windef->frameOptions != FRAMEOPTION_DEFAULTS)
+				ereport(ERROR,
+						(errcode(ERRCODE_WINDOWING_ERROR),
+						 errmsg("cannot copy window \"%s\" because it has a frame clause",
+								windef->refname),
+						 parser_errposition(pstate, windef->location)));
+			/* Else this clause is just OVER (foo), so say this: */
 			ereport(ERROR,
 					(errcode(ERRCODE_WINDOWING_ERROR),
-					 errmsg("cannot override frame clause of window \"%s\"",
-							windef->refname),
+			errmsg("cannot copy window \"%s\" because it has a frame clause",
+				   windef->refname),
+					 errhint("Omit the parentheses in this OVER clause."),
 					 parser_errposition(pstate, windef->location)));
+		}
 		wc->frameOptions = windef->frameOptions;
 		/* Process frame offset expressions */
 		wc->startOffset = transformFrameOffset(pstate, wc->frameOptions,