diff --git a/src/draw.cpp b/src/draw.cpp
index 7562018..47d0420 100644
--- a/src/draw.cpp
+++ b/src/draw.cpp
@@ -716,6 +716,28 @@ nogrid:;
         SK.constraint.elem[i].Draw();
     }
 
+    // Draw the "pending" constraint, i.e. a constraint that would be
+    // placed on a line that is almost horizontal or vertical
+    if(SS.GW.pending.operation == DRAGGING_NEW_LINE_POINT) {
+        SuggestedConstraint suggested =
+            SS.GW.SuggestLineConstraint(SS.GW.pending.request);
+        if(suggested != GraphicsWindow::SUGGESTED_NONE) {
+            Constraint c;
+            ZERO(&c);
+            c.group = SS.GW.activeGroup;
+            c.workplane = SS.GW.ActiveWorkplane();
+            c.type = suggested;
+            c.ptA = Entity::NO_ENTITY;
+            c.ptB = Entity::NO_ENTITY;
+            c.entityA = SS.GW.pending.request.entity(0);
+            c.entityB = Entity::NO_ENTITY;
+            c.other = false;
+            c.other2 = false;
+            // Only draw.
+            c.Draw();
+        }
+    }
+
     // Draw the traced path, if one exists
     ssglLineWidth(Style::Width(Style::ANALYZE));
     ssglColorRGB(Style::Color(Style::ANALYZE));
diff --git a/src/graphicswin.cpp b/src/graphicswin.cpp
index fef7f2b..ac44d8b 100644
--- a/src/graphicswin.cpp
+++ b/src/graphicswin.cpp
@@ -1016,3 +1016,27 @@ void GraphicsWindow::ToggleBool(bool *v) {
     SS.ScheduleShowTW();
 }
 
+GraphicsWindow::SuggestedConstraint GraphicsWindow::SuggestLineConstraint(hRequest request) {
+    if(LockedInWorkplane()) {
+        Entity *ptA = SK.GetEntity(request.entity(1)),
+               *ptB = SK.GetEntity(request.entity(2));
+
+        Expr *au, *av, *bu, *bv;
+
+        ptA->PointGetExprsInWorkplane(ActiveWorkplane(), &au, &av);
+        ptB->PointGetExprsInWorkplane(ActiveWorkplane(), &bu, &bv);
+
+        Expr *du = au->Minus(bu);
+        Expr *dv = av->Minus(bv);
+
+        const double TOLERANCE = 10.0;
+        if(fabs(du->Eval()) * scale < TOLERANCE)
+            return SUGGESTED_VERTICAL;
+        else if(fabs(dv->Eval()) * scale < TOLERANCE)
+            return SUGGESTED_HORIZONTAL;
+        else
+            return SUGGESTED_NONE;
+    } else {
+        return SUGGESTED_NONE;
+    }
+}
diff --git a/src/mouse.cpp b/src/mouse.cpp
index e968147..ef8a4af 100644
--- a/src/mouse.cpp
+++ b/src/mouse.cpp
@@ -497,6 +497,15 @@ void GraphicsWindow::MouseRightUp(double x, double y) {
 
     if(context.active) return;
 
+    if(pending.operation == DRAGGING_NEW_LINE_POINT) {
+        SuggestedConstraint suggested =
+            SS.GW.SuggestLineConstraint(SS.GW.pending.request);
+        if(suggested != SUGGESTED_NONE) {
+            Constraint::Constrain(suggested,
+                Entity::NO_ENTITY, Entity::NO_ENTITY, pending.request.entity(0));
+        }
+    }
+
     if(pending.operation == DRAGGING_NEW_LINE_POINT ||
        pending.operation == DRAGGING_NEW_CUBIC_POINT)
     {
@@ -806,6 +815,7 @@ void GraphicsWindow::MouseLeftDown(double mx, double my) {
             ClearSuper();
 
             pending.operation = DRAGGING_NEW_LINE_POINT;
+            pending.request = hr;
             pending.point = hr.entity(2);
             pending.description = "click next point of line, or press Esc";
             SK.GetEntity(pending.point)->PointForceTo(v);
@@ -1009,6 +1019,14 @@ void GraphicsWindow::MouseLeftDown(double mx, double my) {
         }
 
         case DRAGGING_NEW_LINE_POINT: {
+            // Constrain the line segment horizontal or vertical if close enough
+            SuggestedConstraint suggested =
+                SS.GW.SuggestLineConstraint(SS.GW.pending.request);
+            if(suggested != SUGGESTED_NONE) {
+                Constraint::Constrain(suggested,
+                    Entity::NO_ENTITY, Entity::NO_ENTITY, pending.request.entity(0));
+            }
+
             if(hover.entity.v) {
                 Entity *e = SK.GetEntity(hover.entity);
                 if(e->IsPoint()) {
@@ -1044,6 +1062,7 @@ void GraphicsWindow::MouseLeftDown(double mx, double my) {
 
             // And drag an endpoint of the new line segment
             pending.operation = DRAGGING_NEW_LINE_POINT;
+            pending.request = hr;
             pending.point = hr.entity(2);
             pending.description = "click next point of line, or press Esc";
 
diff --git a/src/ui.h b/src/ui.h
index 65a4a26..f176219 100644
--- a/src/ui.h
+++ b/src/ui.h
@@ -523,6 +523,7 @@ public:
     struct {
         int             operation;
 
+        hRequest        request;
         hEntity         point;
         List<hEntity>   points;
         hEntity         circle;
@@ -535,6 +536,13 @@ public:
     // The constraint that is being edited with the on-screen textbox.
     hConstraint constraintBeingEdited;
 
+    enum SuggestedConstraint {
+        SUGGESTED_NONE = 0,
+        SUGGESTED_HORIZONTAL = Constraint::HORIZONTAL,
+        SUGGESTED_VERTICAL = Constraint::VERTICAL,
+    };
+    SuggestedConstraint SuggestLineConstraint(hRequest lineSegment);
+
     Vector SnapToGrid(Vector p);
     bool ConstrainPointByHovered(hEntity pt);
     void DeleteTaggedRequests(void);