diff --git a/command_prompt.py b/command_prompt.py
index 2b112e0..b1cd71d 100644
--- a/command_prompt.py
+++ b/command_prompt.py
@@ -313,18 +313,25 @@ class Command_prompt(Label):
 
     # set properties of particles
     def run_set(self,argv):
-        if len(argv)<3:
+        if len(argv)<2:
             self.message="error: 'set' command was run with too few arguments -- usage: 'set <property> <value>'"
             return
 
         if argv[1]=="color":
             self.run_set_color(argv)
             return
+        elif argv[1]=="grid":
+            self.run_set_grid(argv)
+            return
         else:
             self.message="error: unrecognized command '"+argv[1]+"'"
             return
         
+    # set color
     def run_set_color(self,argv):
+        if len(argv)<3:
+            self.message="error: 'set color' command was run with without anargument -- usage: 'set color <color_descriptor>'"
+            return
         if argv[2]=="blue":
             self.app.painter.set_color((0,0,1))
         elif argv[2]=="green":
@@ -356,6 +363,19 @@ class Command_prompt(Label):
                 return
             self.app.painter.set_color(color)
 
+    # toggle grid
+    def run_set_grid(self,argv):
+        if len(argv)==2:
+            # no argument: set to toggle
+            self.app.painter.set_grid(-1)
+        elif argv[2]=="on" or argv[2]=="1":
+            self.app.painter.set_grid(1)
+        elif argv[2]=="off" or argv[2]=="0":
+            self.app.painter.set_grid(0)
+        else:
+            self.message="error: unrecognized argument '"+argv[2]+"' -- usage 'set grid [on|off]'"
+            
+
 
     # write to file
     def run_write(self,argv):
diff --git a/painter.py b/painter.py
index b8feba6..8546c43 100644
--- a/painter.py
+++ b/painter.py
@@ -2,6 +2,7 @@ import sys
 import math
 from kivy.uix.widget import Widget
 from kivy.core.window import Window
+from kivy.graphics import Color,Line
 
 from point import Point
 from polyomino import Cross
@@ -52,9 +53,30 @@ class Painter(Widget):
     def draw(self):
         self.canvas.clear()
         with self.canvas:
+            # draw grids
+            for particle in self.particles:
+                if particle.grid:
+                    self.draw_grid(particle.squares[0].pos)
+
             for particle in self.particles:
                 particle.draw()
 
+    def draw_grid(self,pos):
+        # height offset due to status bar and command prompt
+        height_offset=self.app.status_bar.height+self.app.command_prompt.height
+        # vertical lines
+        # offest wrt 0
+        offset=(pos.x-0.5)%1
+        for i in range(math.floor(self.width/Square_element.size-offset)+1):
+            Color(1,1,1)
+            Line(points=((i+offset)*Square_element.size,height_offset,(i+offset)*Square_element.size,self.height+height_offset))
+        # horizontal lines
+        # offset wrt 0
+        offset=(pos.y-0.5)%1-height_offset/Square_element.size
+        for i in range(math.floor(self.height/Square_element.size-offset)+1):
+            Color(1,1,1)
+            Line(points=(0,(i+offset)*Square_element.size+height_offset,self.width,(i+offset)*Square_element.size+height_offset))
+
 
     # respond to keyboard
     def on_key_down(self, keyboard, keycode, text, modifiers):
@@ -257,6 +279,19 @@ class Painter(Widget):
         # redraw
         self.draw()
 
+    # set grid for selected particles
+    # onoff: 1:on, 0:off, -1:toggle
+    def set_grid(self,onoff):
+        for particle in self.selected:
+            if onoff==1:
+                particle.grid=True
+            elif onoff==0:
+                particle.grid=False
+            elif onoff==-1:
+                particle.grid=not particle.grid
+        # redraw
+        self.draw()
+
 
     # write configuration to file
     def write(self,file):
diff --git a/polyomino.py b/polyomino.py
index a255856..7bfe366 100644
--- a/polyomino.py
+++ b/polyomino.py
@@ -14,6 +14,9 @@ class Polyomino():
         self.color=kwargs.get("color",(0,0,1))
         self.selected=False
 
+        # whether to draw a background grid
+        self.grid=kwargs.get("grid",False)
+
     # draw function
     def draw(self):
         # set color