diff --git a/src/lattice.py b/src/lattice.py
new file mode 100644
index 0000000..2f06706
--- /dev/null
+++ b/src/lattice.py
@@ -0,0 +1,38 @@
+from point import Point
+
+# parent class of all lattices
+class Lattice():
+    def __init__(self,**kwargs):
+        self.type=kwargs.get("type","")
+
+    # lattice point nearest to point
+    # overwrite in subclasses
+    def nearest(self,point):
+        return point
+
+    # delta to nearest point
+    def nearest_delta(self,point):
+        return self.nearest(point)-point
+
+    # draw lattice
+    # overwrite in subclasses
+    def draw(self,painter):
+        return
+
+
+# square lattice
+class Square_lattice(Lattice):
+    def __init__(self,**kwargs):
+
+        self.spacing=kwargs.get("spacing",1.)
+
+        super(Square_lattice,self).__init__(**kwargs,type="square")
+
+    # lattice point nearest to point
+    def nearest(self,point):
+        return Point(round(point.x/self.spacing)*self.spacing,round(point.y/self.spacing)*self.spacing)
+
+    # draw
+    def draw(self,painter):
+        painter.draw_grid(Point(0,0),self.spacing)
+        
diff --git a/src/painter.py b/src/painter.py
index bd32c0a..8a9fd02 100644
--- a/src/painter.py
+++ b/src/painter.py
@@ -7,6 +7,7 @@ from kivy.graphics import Color,Line
 from point import Point
 from polyomino import Cross
 from polyomino import Square_element
+from lattice import Square_lattice
 
 from tools import remove_fromlist
 
@@ -20,6 +21,10 @@ class Painter(Widget):
         # list of particles
         self.particles=[]
 
+        # underlying lattice
+        #self.lattice=None
+        self.lattice=Square_lattice()
+
         # particle under mouse
         self.undermouse=None
 
@@ -74,6 +79,10 @@ class Painter(Widget):
             for particle in self.particles:
                 particle.draw(self)
 
+            # draw lattice
+            if self.lattice!=None:
+                self.lattice.draw(self)
+
             # draw grids
             for particle in self.particles:
                 if particle.grid>0:
@@ -175,6 +184,10 @@ class Painter(Widget):
             # create new cross
             if touch.button=="right":
                 new=Cross(touchx,touchy)
+                # snap to lattice
+                if self.lattice!=None:
+                    new.move(self.lattice.nearest_delta(new.squares[0].pos))
+
                 if not self.check_interaction_any(new,Point(0,0)):
                     # add to list
                     self.particles.append(new)
@@ -237,18 +250,30 @@ class Painter(Widget):
 
     # respond to drag
     def on_touch_move(self,touch):
-        # convert to logical
-        touchx=self.coord_topos_x(touch.x)
-        touchy=self.coord_topos_y(touch.y)
 
         # only respond to touch in drawing area
         if self.collide_point(*touch.pos):
+            # convert to logical
+            touchx=self.coord_topos_x(touch.x)
+            touchy=self.coord_topos_y(touch.y)
+
             # only move on left click
             if touch.button=="left" and self.modifiers==[] and self.undermouse!=None:
                 # attempted move determined by the relative position to the relative position of click within self.undermouse
                 delta=self.adjust_move(Point(touchx,touchy)-(self.offset+self.undermouse.squares[0].pos),0)
-                for particle in self.selected:
-                    particle.move(delta)
+
+                # snap to lattice
+                if self.lattice!=None:
+                    delta=self.lattice.nearest(delta)
+                    # check that the move is possible (which is not guaranteed after snapping to lattice)
+                    if not self.check_interaction_unselected_list(self.selected,delta):
+                        for particle in self.selected:
+                            particle.move(delta)
+
+                # no lattice, move is guaranteed to be acceptable
+                else:
+                    for particle in self.selected:
+                        particle.move(delta)
 
                 # redraw
                 self.draw()
diff --git a/src/polyomino.py b/src/polyomino.py
index 8a96de8..6a79106 100644
--- a/src/polyomino.py
+++ b/src/polyomino.py
@@ -8,7 +8,7 @@ from tools import isint_nonzero,sgn,in_interval
 # parent class of all polyominos
 class Polyomino():
     def __init__(self,**kwargs):
-        # square elements that maje up the polyomino 
+        # square elements that make up the polyomino 
         self.squares=kwargs.get("squares",[])
 
         self.color=kwargs.get("color",(0,0,1))