diff --git a/dotscape b/dotscape
index dd3e33a..320a3a3 100755
Binary files a/dotscape and b/dotscape differ
diff --git a/functional-core/app-type.sml b/functional-core/app-type.sml
index 59282ec..bfe5ef8 100644
--- a/functional-core/app-type.sml
+++ b/functional-core/app-type.sml
@@ -23,7 +23,7 @@ sig
     , xClickPoints: Real32.real vector
     , yClickPoints: Real32.real vector
     , graphLines: Real32.real vector
-    , undo: (int * int) list
+    , undo: (Real32.real * Real32.real) list
     }
 end
 
@@ -60,6 +60,6 @@ struct
     , xClickPoints: Real32.real vector
     , yClickPoints: Real32.real vector
     , graphLines: Real32.real vector
-    , undo: (int * int) list
+    , undo: (Real32.real * Real32.real) list
     }
 end
diff --git a/functional-core/app-update.sml b/functional-core/app-update.sml
index 5d54789..1b06f29 100644
--- a/functional-core/app-update.sml
+++ b/functional-core/app-update.sml
@@ -44,6 +44,7 @@ struct
         , windowWidth
         , windowHeight
         )
+      val newUndoTuple = (hpos, vpos)
     in
       if Vector.length buttonVec > 0 then
         case #triangleStage model of
@@ -53,7 +54,8 @@ struct
               val drawMsg = DRAW_BUTTON drawVec
 
               val newTriangleStage = FIRST {x1 = hpos, y1 = vpos}
-              val model = AppWith.triangleStage (model, newTriangleStage)
+              val model =
+                AppWith.newTriangleStage (model, newTriangleStage, newUndoTuple)
             in
               (model, drawMsg, mouseX, mouseY)
             end
@@ -65,14 +67,15 @@ struct
 
               val newTriangleStage = SECOND
                 {x1 = x1, y1 = y1, x2 = hpos, y2 = vpos}
-              val model = AppWith.triangleStage (model, newTriangleStage)
+              val model =
+                AppWith.newTriangleStage (model, newTriangleStage, newUndoTuple)
             in
               (model, drawMsg, mouseX, mouseY)
             end
         | SECOND {x1, y1, x2, y2} =>
             let
               val model = AppWith.newTriangle
-                (model, x1, y1, x2, y2, hpos, vpos)
+                (model, x1, y1, x2, y2, hpos, vpos, newUndoTuple)
 
               val drawVec = Triangles.toVector model
               val drawMsg = DRAW_TRIANGLES_AND_RESET_BUTTONS drawVec
@@ -95,6 +98,49 @@ struct
       (model, drawMsg, mouseX, mouseY)
     end
 
+  fun undoAction (model, mouseX, mouseY) =
+    case #triangleStage model of
+      FIRST {x1, y1} =>
+        (* Change FIRST to NO_TRIANGLE and clear buttons. *)
+        let val model = AppWith.replaceTriangleStage (model, NO_TRIANGLE)
+        in (model, CLEAR_BUTTONS, mouseX, mouseY)
+        end
+    | SECOND {x1, y1, x2, y2} =>
+        (* Change FIRST to SECOND and redraw buttons. *)
+        let
+          val newTriangleStage = FIRST {x1 = x1, y1 = y1}
+          val model = AppWith.replaceTriangleStage (model, newTriangleStage)
+
+          val drawVec =
+            TriangleStage.firstToVector (x1, y1, Vector.fromList [], model)
+          val drawMsg = DRAW_BUTTON drawVec
+        in
+          (model, drawMsg, mouseX, mouseY)
+        end
+    | NO_TRIANGLE =>
+        (case #triangles model of
+           {x1, y1, x2, y2, ...} :: trianglesTl =>
+             (* Have to slice off (x3, y3) from triangle head,
+              * turn (x1, y1, x2, y2) into a triangleStage,
+              * and redraw both triangle and triangleStage. *)
+             let
+               val triangleStage = SECOND {x1 = x1, y1 = y1, x2 = x2, y2 = y2}
+               val model =
+                 AppWith.undoTriangle (model, triangleStage, trianglesTl)
+
+               val newTriangleVec = Triangles.toVector model
+               val drawVec = TriangleStage.secondToVector
+                 (x1, y1, x2, y2, newTriangleVec, model)
+               val drawMsg =
+                 DRAW_TRIANGLES_AND_BUTTONS
+                   {triangles = newTriangleVec, buttons = drawVec}
+             in
+               (model, drawMsg, mouseX, mouseY)
+             end
+         | [] =>
+             (* Can't undo, because there are no actions to undo. *)
+             (model, NO_DRAW, mouseX, mouseY))
+
   fun update (model: app_type, mouseX, mouseY, inputMsg) =
     case inputMsg of
       MOUSE_MOVE {x = mouseX, y = mouseY} =>
@@ -103,10 +149,5 @@ struct
     | MOUSE_LEFT_CLICK => mouseLeftClick (model, mouseX, mouseY)
     | RESIZE_WINDOW {width, height} =>
         resizeWindow (model, mouseX, mouseY, width, height)
-    | UNDO_ACTION =>
-        let
-          val _ = print "undo action\n"
-        in
-          (model, NO_DRAW, mouseX, mouseY)
-        end
+    | UNDO_ACTION => undoAction (model, mouseX, mouseY)
 end
diff --git a/functional-core/app-with.sml b/functional-core/app-with.sml
index f1e56ca..d95d720 100644
--- a/functional-core/app-with.sml
+++ b/functional-core/app-with.sml
@@ -1,8 +1,17 @@
 signature APP_WITH =
 sig
   val windowResize: AppType.app_type * int * int -> AppType.app_type
-  val triangleStage: AppType.app_type * AppType.triangle_stage
-                     -> AppType.app_type
+
+  val newTriangleStage:
+    AppType.app_type * AppType.triangle_stage * (Real32.real * Real32.real)
+    -> AppType.app_type
+  val replaceTriangleStage: AppType.app_type * AppType.triangle_stage
+                            -> AppType.app_type
+
+  val undoTriangle:
+    AppType.app_type * AppType.triangle_stage * AppType.triangle list
+    -> AppType.app_type
+
   val newTriangle:
     AppType.app_type
     * Real32.real
@@ -11,6 +20,7 @@ sig
     * Real32.real
     * Real32.real
     * Real32.real
+    * (Real32.real * Real32.real)
     -> AppType.app_type
 end
 
@@ -18,7 +28,33 @@ structure AppWith :> APP_WITH =
 struct
   open AppType
 
-  fun triangleStage (app: app_type, newTriangleStage: triangle_stage) : app_type =
+  fun newTriangleStage
+    (app: app_type, newTriangleStage: triangle_stage, xyTuple) : app_type =
+    let
+      val
+        { triangleStage = _
+        , triangles
+        , xClickPoints
+        , yClickPoints
+        , windowWidth
+        , windowHeight
+        , graphLines
+        , undo
+        } = app
+      val newUndo = xyTuple :: undo
+    in
+      { triangleStage = newTriangleStage
+      , triangles = triangles
+      , xClickPoints = xClickPoints
+      , yClickPoints = yClickPoints
+      , windowWidth = windowWidth
+      , windowHeight = windowHeight
+      , graphLines = graphLines
+      , undo = newUndo
+      }
+    end
+
+  fun replaceTriangleStage (app: app_type, newTriangleStage) =
     let
       val
         { triangleStage = _
@@ -42,7 +78,33 @@ struct
       }
     end
 
-  fun newTriangle (app: app_type, x1, y1, x2, y2, x3, y3) : app_type =
+  fun undoTriangle
+    (app: app_type, newTriangleStage: triangle_stage, trianglesTl) : app_type =
+    let
+      val
+        { triangleStage = _
+        , triangles = _
+        , xClickPoints
+        , yClickPoints
+        , windowWidth
+        , windowHeight
+        , graphLines
+        , undo
+        } = app
+    in
+      { triangleStage = newTriangleStage
+      , triangles = trianglesTl
+      , xClickPoints = xClickPoints
+      , yClickPoints = yClickPoints
+      , windowWidth = windowWidth
+      , windowHeight = windowHeight
+      , graphLines = graphLines
+      , undo = undo
+      }
+    end
+
+  fun newTriangle (app: app_type, x1, y1, x2, y2, x3, y3, newUndoTuple) :
+    app_type =
     let
       val
         { triangles
@@ -57,6 +119,7 @@ struct
 
       val newTriangle = {x1 = x1, y1 = y1, x2 = x2, y2 = y2, x3 = x3, y3 = y3}
       val newTriangles = newTriangle :: triangles
+      val newUndo = newUndoTuple :: undo
     in
       { triangleStage = NO_TRIANGLE
       , triangles = newTriangles
@@ -65,7 +128,7 @@ struct
       , windowWidth = windowWidth
       , windowHeight = windowHeight
       , graphLines = graphLines
-      , undo = undo
+      , undo = newUndo
       }
     end
 
diff --git a/imperative-shell/event-loop.sml b/imperative-shell/event-loop.sml
index f81f0bd..0b070e8 100644
--- a/imperative-shell/event-loop.sml
+++ b/imperative-shell/event-loop.sml
@@ -91,6 +91,44 @@ struct
                    , triangleDrawLength
                    )
                end
+           | DRAW_TRIANGLES_AND_BUTTONS
+               {triangles = triangleVec, buttons = buttonsVec} =>
+               let
+                 val _ =
+                   AppDraw.uploadTrianglesVector
+                     (triangleDrawObject, triangleVec)
+                 val triangleDrawLength = Vector.length triangleVec div 2
+
+                 val _ =
+                   AppDraw.uploadButtonVector (buttonDrawObject, buttonsVec)
+                 val buttonDrawLength = Vector.length buttonsVec div 5
+               in
+                 draw
+                   ( drawMailbox
+                   , window
+                   , graphDrawObject
+                   , drawGraphLength
+                   , buttonDrawObject
+                   , buttonDrawLength
+                   , triangleDrawObject
+                   , triangleDrawLength
+                   )
+               end
+           | CLEAR_BUTTONS =>
+               let
+                 val buttonDrawLength = 0
+               in
+                 draw
+                   ( drawMailbox
+                   , window
+                   , graphDrawObject
+                   , drawGraphLength
+                   , buttonDrawObject
+                   , buttonDrawLength
+                   , triangleDrawObject
+                   , triangleDrawLength
+                   )
+               end
            | RESIZE_TRIANGLES_BUTTONS_AND_GRAPH {triangles, graphLines} =>
                let
                  val _ =
diff --git a/message-types/draw-msg.sml b/message-types/draw-msg.sml
index d33916b..4c048c0 100644
--- a/message-types/draw-msg.sml
+++ b/message-types/draw-msg.sml
@@ -2,9 +2,12 @@ signature DRAW_MESSAGE =
 sig
   datatype t =
     DRAW_BUTTON of Real32.real vector
+  | DRAW_TRIANGLES_AND_BUTTONS of
+      {triangles: Real32.real vector, buttons: Real32.real vector}
   | DRAW_TRIANGLES_AND_RESET_BUTTONS of Real32.real vector
   | RESIZE_TRIANGLES_BUTTONS_AND_GRAPH of
       {triangles: Real32.real vector, graphLines: Real32.real vector}
+  | CLEAR_BUTTONS
   | NO_DRAW
 end
 
@@ -12,8 +15,11 @@ structure DrawMessage :> DRAW_MESSAGE =
 struct
   datatype t =
     DRAW_BUTTON of Real32.real vector
+  | DRAW_TRIANGLES_AND_BUTTONS of
+      {triangles: Real32.real vector, buttons: Real32.real vector}
   | DRAW_TRIANGLES_AND_RESET_BUTTONS of Real32.real vector
   | RESIZE_TRIANGLES_BUTTONS_AND_GRAPH of
       {triangles: Real32.real vector, graphLines: Real32.real vector}
+  | CLEAR_BUTTONS
   | NO_DRAW
 end