diff --git a/polyomino.py b/polyomino.py
index 6a52b0f..5196d72 100644
--- a/polyomino.py
+++ b/polyomino.py
@@ -5,21 +5,17 @@ from kivy.graphics import Color,Line,Rectangle
 from point import Point,l_infinity
 from tools import isint_nonzero,sgn,in_interval
 
-class Square():
-    # size
-    size=50
+# parent class of all polyominos
+class Polyomino():
+    def __init__(self,**kwargs):
+        # square elements that maje up the polyomino 
+        self.squares=kwargs.get("squares",[])
 
-    def __init__(self,x,y,**kwargs):
-        self.pos=Point(x,y)
         self.color=kwargs.get("color",(0,0,1))
         self.selected=False
 
-    # set position
-    def setpos(self,x,y):
-        self.pos.x=x
-        self.pos.y=y
-
-    def draw(self,**kwargs):
+    # draw function
+    def draw(self):
         # set color
         if not self.selected:
             Color(*self.color)
@@ -28,7 +24,45 @@ class Square():
             # darken selected
             Color(r/2,g/2,b/2)
 
-        Rectangle(pos=((self.pos.x-0.5)*self.size,(self.pos.y-0.5)*self.size),size=(self.size,self.size))
+        for square in self.squares:
+            Rectangle(pos=((square.pos.x-0.5)*square.size,(square.pos.y-0.5)*square.size),size=(square.size,square.size))
+
+        # draw boundary
+        self.stroke()
+
+    # draw boundary (override for connected polyominos)
+    def stroke(self):
+        # white
+        Color(1,1,1)
+        for square in self.squares:
+            Line(points=(
+            *((square.pos.x-0.5)*square.size,(square.pos.y-0.5)*square.size),
+            *((square.pos.x-0.5)*square.size,(square.pos.y+0.5)*square.size),
+            *((square.pos.x+0.5)*square.size,(square.pos.y+0.5)*square.size),
+            *((square.pos.x+0.5)*square.size,(square.pos.y-0.5)*square.size)
+            ))
+        
+
+# square
+class Square(Polyomino):
+    def __init__(self,x,y,**kwargs):
+        super(Square,self).__init__(squares=[Square_element(x,y)],kwargs)
+
+
+
+# square building block of polyominos
+class Square_element():
+    # size
+    size=50
+
+    def __init__(self,x,y,**kwargs):
+        self.pos=Point(x,y)
+
+    # set position
+    def setpos(self,x,y):
+        self.pos.x=x
+        self.pos.y=y
+
 
     # check whether a square at pos interacts with square
     def check_interaction(self,pos):