diff --git a/src/corners.py b/src/corners.py
--- a/src/corners.py
+++ b/src/corners.py
@@ -5,6 +5,7 @@ class Corners:
   def __init__(self):
     self.corners=[]
   
+  ## Adds a new corner if there are less than four, replaces the closest otherwise.
   def add(self,x,y):
     a=EPoint(x,y)
     # for i,c in enumerate(self.corners): # move an improperly placed point
@@ -24,15 +25,15 @@ class Corners:
         index,minDist=i,a.dist(c)
     
     self.corners[index]=a
-    self._canonizeOrder()
   
   
-  ## Computes twice the area of a triangle formed by points a,b,c.
+  ## Computes twice the area of the triangle formed by points a,b,c.
   #  
   #  @return positive value for points oriented counter-clockwise, negative for clockwise, zero for degenerate cases.
   def _doubleTriangleArea(a,b,c):
     return (a.x-b.x)*(c.y-a.y)-(c.x-a.x)*(a.y-b.y)
   
+  
   def _slope(a,b):
     if(b.x==a.x): return float("inf")
     return (b.y-a.y)/(b.x-a.x)
@@ -41,7 +42,7 @@ class Corners:
   ## Order the corners (0,1,2,3) so they make a quadrangle with vertices KLMN in counter-clockwise order, K being in the upper left.
   #  
   #  For four points ABCD, there are 24 possible permutations corresponding to the desired KLMN.
-  #  When we relax the condition of K being the upper left one, we get six groups of four equivalent permuations. KLMN ~ LMNK ~ MNKL ~ NKLM.
+  #  When we relax the condition of K being the upper left one, we get six groups of four equivalent permutations. KLMN ~ LMNK ~ MNKL ~ NKLM.
   #  
   #  xxxx -> KLMN | ABC | ABD | ACD | BCD | index | swap
   #  ------------ | :-: | :-: | :-: | :-: | ----: | ----
@@ -55,11 +56,11 @@ class Corners:
   #  For every non-degenerate quadrangle, there must be 1-3 edges going right-left (from a higher to a lower x coordinate).
   #  From these pick the one with the lowest slope (dy/dx) and declare its ending point the upper left corner. For the same slope pick the one further left.
   #  
-  #  @return True for a convex quadrangle, False for concave or degenerate cases.
-  def _canonizeOrder(self):
+  #  @return True for a convex quadrangle, False for concave and degenerate cases.
+  def canonizeOrder(self):
     if len(self.corners)!=4: return False # erroneus call
     
-    a,b,c,d=(x for x in self.corners)
+    a,b,c,d=self.corners
     abc=Corners._doubleTriangleArea(a,b,c)
     abd=Corners._doubleTriangleArea(a,b,d)
     acd=Corners._doubleTriangleArea(a,c,d)
@@ -93,5 +94,5 @@ class Corners:
 # for perm in itertools.permutations(points):
   # corn=Corners()
   # for p in perm: corn.add(p[0],p[1])
-  # print(corn._canonizeOrder())
+  # print(corn.canonizeOrder())
   # print(corn.corners)
diff --git a/src/gui.py b/src/gui.py
--- a/src/gui.py
+++ b/src/gui.py
@@ -3,11 +3,14 @@ from PIL import ImageTk
 import PIL
 import math
 from epoint import *
+from corners import *
+from grid import *
 
 
 class Application(tk.Frame):
   def __init__(self, master=None):
-    self.corners=[]
+    self.corners=Corners()
+    self.boardGrid=None
     
     tk.Frame.__init__(self, master)
     self.grid(column=0,row=0)
@@ -31,35 +34,16 @@ class Application(tk.Frame):
   
   ## Stores a grid corner located at x,y coordinates.
   def addCorner(self,x,y):
-    a=EPoint(x,y)
-    for i,c in enumerate(self.corners): # move an improperly placed point
-      if a.dist(c)<20:
-        self.corners[i]=a
-        # print(self.corners)
-        self.redrawImgView()
-        return
-    
-    if len(self.corners)<4: # add a new corner
-      self.corners.append(a)
-      # print(self.corners)
-      self.redrawImgView()
-    
-    if len(self.corners)<4: return
-    
-    index,minDist=0,float('inf') # replace the corner closest to the clicked point
-    for i,c in enumerate(self.corners):
-      if a.dist(c)<minDist:
-        index,minDist=i,a.dist(c)
-    
-    self.corners[index]=a
-    # print(self.corners)
+    self.corners.add(x,y)
+    if self.corners.canonizeOrder():
+      self.boardGrid=Grid(self.corners)
     
     self.redrawImgView()
     
   ## Redraws the current image and its overlay.
   def redrawImgView(self):
     self.imgView.create_image(2,2,anchor="nw",image=self.img)
-    for corner in self.corners:
+    for corner in self.corners.corners:
       self.markPoint(corner.x,corner.y)
     
     self.imgView.grid()