diff --git a/devel.but b/devel.but
index a17d3b9..afc7fb1 100644
--- a/devel.but
+++ b/devel.but
@@ -1056,11 +1056,16 @@ puzzle's drawing area.
 pointer will be to read the game's tile size parameter in order to
 divide mouse coordinates by it.)
 
-\cw{interpret_move()} may return in three different ways:
+\cw{interpret_move()} may return in four different ways:
 
-\b Returning \cw{NULL} indicates that no action whatsoever occurred
-in response to the input event; the puzzle was not interested in it
-at all.
+\b Returning \cw{MOVE_UNUSED} or \cw{MOVE_NO_EFFECT} indicates that no
+action whatsoever occurred in response to the input event; the puzzle
+was not interested in it at all.  The distinction between this is that
+\cw{MOVE_NO_EFFECT} implies that the state of the game is what makes
+the event uninteresting, while \cw{MOVE_NO_EFFECT} means that the
+event is intrinsically uninteresting.  For example, a mouse click on
+an already-revealed square in Mines might return \cw{MOVE_NO_EFFECT}
+while a click outside the board would return \cw{MOVE_UNUSED}.
 
 \b Returning the special value \cw{MOVE_UI_UPDATE} indicates that the input
 event has resulted in a change being made to the \c{game_ui} which
@@ -1080,7 +1085,8 @@ strings can be written to disk when saving the game and fed to
 
 The return value from \cw{interpret_move()} is expected to be
 dynamically allocated if and only if it is not either \cw{NULL}
-\e{or} the special string constant \c{MOVE_UI_UPDATE}.
+\e{or} one of the special string constants \cw{MOVE_UNUSED},
+\cw{MOVE_NO_EFFECT}, or \cw{MOVE_UI_UPDATE}.
 
 After this function is called, the back end is permitted to rely on
 some subsequent operations happening in sequence:
diff --git a/midend.c b/midend.c
index 5de84c0..1064af3 100644
--- a/midend.c
+++ b/midend.c
@@ -990,7 +990,7 @@ static bool midend_really_process_key(midend *me, int x, int y, int button,
             me->ui, me->drawstate, x, y, button);
     }
 
-    if (!movestr) {
+    if (movestr == NULL || movestr == MOVE_UNUSED) {
 	if ((me->one_key_shortcuts && (button == 'n' || button == 'N')) ||
              button == '\x0E' || button == UI_NEWGAME) {
 	    midend_new_game(me);
@@ -1025,6 +1025,8 @@ static bool midend_really_process_key(midend *me, int x, int y, int button,
 	    goto done;
 	} else
 	    goto done;
+    } else if (movestr == MOVE_NO_EFFECT) {
+        goto done;
     } else {
         *handled = true;
 	if (movestr == MOVE_UI_UPDATE)
diff --git a/misc.c b/misc.c
index 341b5ac..9a757d4 100644
--- a/misc.c
+++ b/misc.c
@@ -16,6 +16,8 @@
 #include "puzzles.h"
 
 char MOVE_UI_UPDATE[] = "";
+char MOVE_NO_EFFECT[] = "";
+char MOVE_UNUSED[] = "";
 
 void free_cfg(config_item *cfg)
 {
diff --git a/puzzles.h b/puzzles.h
index 0222f8b..25df27b 100644
--- a/puzzles.h
+++ b/puzzles.h
@@ -800,12 +800,20 @@ extern const game thegame;
 #endif
 
 /*
- * Special string value to return from interpret_move in the case
- * where the game UI has been updated but no actual move is being
- * appended to the undo chain. Must be declared as a non-const char,
- * but should never actually be modified by anyone.
+ * Special string values to return from interpret_move.
+ *
+ * MOVE_UI_UPDATE is for the case where the game UI has been updated
+ * but no actual move is being appended to the undo chain.
+ *
+ * MOVE_NO_EFFECT is for when the key was understood by the puzzle,
+ * but it happens that there isn't effect, not even a UI change.
+ *
+ * MOVE_UNUSED is for keys that the puzzle has no use for at all.
+ *
+ * Each must be declared as a non-const char, but should never
+ * actually be modified by anyone.
  */
-extern char MOVE_UI_UPDATE[];
+extern char MOVE_UI_UPDATE[], MOVE_NO_EFFECT[], MOVE_UNUSED[];
 
 /* A little bit of help to lazy developers */
 #define DEFAULT_STATUSBAR_TEXT "Use status_bar() to fill this in."