diff --git a/src/Mod/Part/App/Attacher.cpp b/src/Mod/Part/App/Attacher.cpp
index 0493014176..f4e8b31476 100644
--- a/src/Mod/Part/App/Attacher.cpp
+++ b/src/Mod/Part/App/Attacher.cpp
@@ -247,7 +247,8 @@ Base::Placement AttachEngine::placementFactory(const gp_Dir &ZAxis,
 
 eMapMode AttachEngine::listMapModes(eSuggestResult& msg,
                                     std::vector<eMapMode>* allApplicableModes,
-                                    std::set<eRefType>* nextRefTypeHint) const
+                                    std::set<eRefType>* nextRefTypeHint,
+                                    std::set<eMapMode>* reachableModes) const
 {
     //replace a pointer with a valid reference, to avoid checks for zero pointer everywhere
     std::vector<eMapMode> buf;
@@ -263,6 +264,12 @@ eMapMode AttachEngine::listMapModes(eSuggestResult& msg,
     std::set<eRefType> &hints = *nextRefTypeHint;
     hints.clear();
 
+    std::set<eMapMode> buf3;
+    if (reachableModes == 0)
+        reachableModes = &buf3;
+    std::set<eMapMode> &mlist_reachable = *reachableModes;
+    mlist_reachable.clear();
+
 
     std::vector<App::GeoFeature*> parts;
     std::vector<const TopoDS_Shape*> shapes;
@@ -305,8 +312,10 @@ eMapMode AttachEngine::listMapModes(eSuggestResult& msg,
                 }
             }
 
-            if (score > 0  &&  str.size() > typeStr.size())
+            if (score > 0  &&  str.size() > typeStr.size()){
                 hints.insert(str[typeStr.size()]);
+                reachableModes->insert(eMapMode(iMode));
+            }
 
             //size check is last, because we needed to collect hints
             if (str.size() != typeStr.size())
diff --git a/src/Mod/Part/App/Attacher.h b/src/Mod/Part/App/Attacher.h
index 1f380c7773..d69abfd964 100644
--- a/src/Mod/Part/App/Attacher.h
+++ b/src/Mod/Part/App/Attacher.h
@@ -216,10 +216,13 @@ public: //methods
      * right type.
      *
      * @param nextRefTypeHint (output). A hint of what can be added to references.
+     *
+     * @param reachableModes (output). List of modes that can be reached by selecing more references.
      */
     virtual eMapMode listMapModes(eSuggestResult &msg,
                                   std::vector<eMapMode>* allApplicableModes = 0,
-                                  std::set<eRefType>* nextRefTypeHint = 0) const;
+                                  std::set<eRefType>* nextRefTypeHint = 0,
+                                  std::set<eMapMode> *reachableModes = 0) const;
 
     /**
      * @brief getHint function returns a set of types that user can add to
diff --git a/src/Mod/PartDesign/Gui/TaskDatumParameters.cpp b/src/Mod/PartDesign/Gui/TaskDatumParameters.cpp
index 5f54140ade..8fb79507dd 100644
--- a/src/Mod/PartDesign/Gui/TaskDatumParameters.cpp
+++ b/src/Mod/PartDesign/Gui/TaskDatumParameters.cpp
@@ -675,9 +675,16 @@ void TaskDatumParameters::updateListOfModes(eMapMode curMode)
     //obtain list of available modes:
     Part::Datum* pcDatum = static_cast<Part::Datum*>(DatumView->getObject());
     eMapMode suggMode = mmDeactivated;
+    std::set<eMapMode> reachableModes;
+    int lastValidModeItemIndex = mmDummy_NumberOfModes;
     if (pcDatum->Support.getSize() > 0){
         eSuggestResult msg;
-        suggMode = pcDatum->attacher().listMapModes(msg, &modesInList);
+        suggMode = pcDatum->attacher().listMapModes(msg, &modesInList, 0, &reachableModes);
+        //add reachable modes to the list, too, but gray them out (using lastValidModeItemIndex, later)
+        lastValidModeItemIndex = modesInList.size()-1;
+        for(eMapMode m: reachableModes){
+            modesInList.push_back(m);
+        }
     } else {
         //no references - display all modes
         modesInList.clear();
@@ -702,13 +709,17 @@ void TaskDatumParameters::updateListOfModes(eMapMode curMode)
                              AttacherGui::getRefListForMode(pcDatum->attacher(),mmode).join(QString::fromLatin1("\n")));
             if (mmode == curMode)
                 iSelect = ui->listOfModes->item(i);
-            if (mmode == suggMode){
-                //make it bold
+            if (i > lastValidModeItemIndex){
+                //potential mode - can be reached by selecting more stuff
+                item->setFlags(item->flags() & ~(Qt::ItemFlag::ItemIsEnabled | Qt::ItemFlag::ItemIsSelectable));
+            } else if (mmode == suggMode){
+                //suggested mode - make bold
                 assert (item);
                 QFont fnt = item->font();
                 fnt.setBold(true);
                 item->setFont(fnt);
             }
+
         }
     }
     //restore selection