diff --git a/hgeometry-examples/duality/Main.hs b/hgeometry-examples/duality/Main.hs
new file mode 100644
index 000000000..4d780eecc
--- /dev/null
+++ b/hgeometry-examples/duality/Main.hs
@@ -0,0 +1,181 @@
+{-# LANGUAGE OverloadedStrings          #-}
+{-# LANGUAGE TemplateHaskell            #-}
+module Main(main) where
+
+import           Control.Lens hiding (view, element)
+import qualified Data.IntMap as IntMap
+import qualified Data.List.NonEmpty as NonEmpty
+import qualified Data.Map as Map
+import           GHC.TypeNats
+import           HGeometry.Box
+import           HGeometry.Duality
+import           HGeometry.Ext
+import           HGeometry.Intersection
+import           HGeometry.Transformation
+import           HGeometry.Line
+import           HGeometry.LineSegment
+import           HGeometry.Viewport
+import           HGeometry.Miso.OrphanInstances ()
+import           HGeometry.Miso.Svg
+import           HGeometry.Miso.Svg.Canvas (Canvas, blankCanvas, mouseCoordinates, theViewport)
+import qualified HGeometry.Miso.Svg.Canvas as Canvas
+import           HGeometry.Number.Real.Rational
+import           HGeometry.Point
+import           HGeometry.Vector
+import qualified Language.Javascript.JSaddle.Warp as JSaddle
+import           Miso
+import           Miso.String (MisoString,ToMisoString(..), ms)
+import           Miso.Svg hiding (height_, id_, style_, width_)
+
+--------------------------------------------------------------------------------
+
+type R = RealNumber 5
+type Color = MisoString
+
+
+data PartialLine = PrimalStart !(Point 2 R)
+                 | DualStart !(Point 2 R)
+                 deriving (Show,Eq)
+
+data Model = Model { _primalCanvas  :: Canvas R
+                   , _dualCanvas    :: Canvas R
+                   , _primalPoints  :: IntMap.IntMap (Point 2 R :+ Color)
+                   , _primalLines   :: IntMap.IntMap (LineEQ R :+ Color)
+                   , _partialLine   :: Maybe (PartialLine :+ Color)
+                   } deriving (Eq)
+makeLenses ''Model
+
+--------------------------------------------------------------------------------
+
+dualPoints :: Getter Model (IntMap.IntMap (Point 2 R :+ Color))
+dualPoints = to $ over (traverse.core) dualPoint . (^.primalLines)
+
+dualLines :: Getter Model (IntMap.IntMap (LineEQ R :+ Color))
+dualLines = to $ over (traverse.core) dualLine . (^.primalPoints)
+
+--------------------------------------------------------------------------------
+
+instance KnownNat p => ToMisoString (RealNumber p) where
+  toMisoString = toMisoString . toFixed
+
+----------------------------------------
+
+initialModel :: Model
+initialModel = Model canvas canvas mempty mempty Nothing
+  where
+    canvas = blankCanvas 576 576 & theViewport.worldToHost %~ (uniformScaling 10 |.|)
+    -- scale world to host by a factor of 10
+
+--------------------------------------------------------------------------------
+
+data Action = Id
+            | PrimalCanvasAction Canvas.InternalCanvasAction
+            | DualCanvasAction Canvas.InternalCanvasAction
+            | PrimalClick
+            | DualClick
+            deriving (Show,Eq)
+
+
+updateModel   :: Model -> Action -> Effect Action Model
+updateModel m = \case
+    Id                    -> noEff m
+    PrimalCanvasAction ca -> m&primalCanvas %%~ flip Canvas.handleInternalCanvasAction ca
+    DualCanvasAction ca   -> m&dualCanvas   %%~ flip Canvas.handleInternalCanvasAction ca
+    PrimalClick           -> noEff addPrimalPoint
+    DualClick             -> noEff addDualPoint
+  where
+    color = "red"
+    addPrimalPoint = case m^.primalCanvas.mouseCoordinates of
+                       Nothing -> m
+                       Just p  -> m&primalPoints %~ insert (p :+ color)
+
+    addDualPoint = case m^.dualCanvas.mouseCoordinates of
+                     Nothing -> m
+                     Just p  -> m&primalLines %~ insert (dualLine p :+ color)
+
+insert     :: p -> IntMap.IntMap p -> IntMap.IntMap p
+insert p m = let k = case IntMap.lookupMax m of
+                            Nothing    -> 0
+                            Just (i,_) -> succ i
+                  in IntMap.insert k p m
+
+--------------------------------------------------------------------------------
+
+viewModel       :: Model -> View Action
+viewModel m = div_ [ ]
+                   [ either PrimalCanvasAction id <$>
+                     Canvas.svgCanvas_ (m^.primalCanvas)
+                                       [ onClick PrimalClick
+                                       , id_ "primalSvg"
+                                       , styleInline_ "border: 1px solid black"
+                                       ]
+                                       primalBody
+                   , either DualCanvasAction id <$>
+                     Canvas.svgCanvas_ (m^.dualCanvas)
+                                       [ onClick DualClick
+                                       , id_ "dualSvg"
+                                       , styleInline_ "border: 1px solid black"
+                                       ]
+                                       dualBody
+                   , div_ []
+                          [text . ms . show $ m^.primalCanvas.mouseCoordinates ]
+                   , div_ []
+                          [text . ms . show $ m^.dualCanvas.mouseCoordinates ]
+                   , div_ []
+                          [text . ms . show $ m^.primalPoints ]
+                   , div_ []
+                          [text . ms . show $ m^.primalLines ]
+                    ]
+  where
+    partialPrimalLine = []
+    partialDualLine = []
+
+    primalBody = drawWorld primalPoints primalLines (m^.primalCanvas.mouseCoordinates)
+    dualBody   = drawWorld dualPoints   dualLines   (m^.dualCanvas.mouseCoordinates)
+
+    drawWorld points lines mousePos =
+         [ g_ [] [ draw p [ fill_ color
+                          ]
+                 ]
+         | p :+ color <- m^..points.folded ]
+      <> [ g_ [] [ draw l [ stroke_ color
+                          ]
+                 ]
+         | l :+ color <- m^..lines.folded ]
+      <> [ draw p [ fill_ "blue" ]  | Just p <- [mousePos] ]
+
+instance Drawable (LineEQ R) where
+  draw l = let maxP = Point2 large large
+               minP = maxP&vector %~ negated
+           in case l `intersect` Rectangle minP maxP of
+                Just (Line_x_Box_Segment s) -> draw s
+                _                           -> flip g_ []
+
+large = 100000
+
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = JSaddle.run 8080 $
+         startApp $
+            App { model         = initialModel
+                , update        = flip updateModel
+                , view          = viewModel
+                , subs          =    Canvas.subs "primalSvg" PrimalCanvasAction
+                                  <> Canvas.subs "dualSvg"   DualCanvasAction
+                , events        = Map.insert "touchstart" False
+                                  . Map.insert "touchmove" False
+                                  . Map.insert "mousemove" False
+                                  $ defaultEvents
+                , initialAction = Id
+                , mountPoint    = Nothing
+                , logLevel      = Off
+                }
+
+textAt                    :: ToMisoString r
+                          => Point 2 r
+                          -> [Attribute action] -> MisoString -> View action
+textAt (Point2 x y) ats t = text_ ([ x_ $ ms x
+                                  , y_ $ ms y
+                                  ] <> ats
+                                  ) [text t]
diff --git a/hgeometry-examples/hgeometry-examples.cabal b/hgeometry-examples/hgeometry-examples.cabal
index 1b3532877..8bbdffb09 100644
--- a/hgeometry-examples/hgeometry-examples.cabal
+++ b/hgeometry-examples/hgeometry-examples.cabal
@@ -134,9 +134,17 @@ executable hgeometry-lineSegmentIntersection
   hs-source-dirs: lineSegmentIntersection
   main-is:        Main.hs
 
+--------------------------------------------------------------------------------
+-- * Duality
+
+executable hgeometry-duality
+  import:         setup, miso-setup
+  hs-source-dirs: duality
+  main-is:        Main.hs
+
 
 --------------------------------------------------------------------------------
--- * LineSegment Intersect
+-- * Voronoi Diagram
 
 executable hgeometry-voronoi
   import:         setup, miso-setup
diff --git a/hgeometry/svg/src/HGeometry/Miso/Svg/Canvas.hs b/hgeometry/svg/src/HGeometry/Miso/Svg/Canvas.hs
index 0c0b291ba..665f367dd 100644
--- a/hgeometry/svg/src/HGeometry/Miso/Svg/Canvas.hs
+++ b/hgeometry/svg/src/HGeometry/Miso/Svg/Canvas.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE OverloadedStrings          #-}
 module HGeometry.Miso.Svg.Canvas
   ( Canvas
+  , theViewport
   , blankCanvas
   , HasDimensions(..)