Secret Codes: The Cardan Grille (“Messages” Unplugged Project)

By , January 10, 2011 2:39 pm

My two oldest children are really into secret codes at the moment and when I asked them for suggestions for this month’s Unplugged Project theme, my 8 year-old son immediately said “Messages! We could make codes!”

So here is our first code, a version of the famous code known as the Cardan Grille.  For this code, a special template is used to encode and decode a message. Here’s how we made ours:

You’ll need paper (graph paper makes it much easier), a sharp pencil, scissors, a ruler, and a box cutter or razor blade.

First mark out evenly spaced boxes on your graph paper.  You will be cutting some of these into square openings, so you’ll have to leave some blank space around each box.  (Our boxes were two graph squares wide and two squares tall with one blank square between rows.)

First we marked off rows of squares then we drew the grid using the ruler.

Next we laminated our paper.  If you don’t have a laminator, you might want to paste it to some stiff cardboard or posterboard just to make it tougher.  If the grid gets lost or destroyed, no one can crack the code and you will be fired as a secret agent!

I used a box cutter to cut random squares out of the grid.  I put an old wooden clipboard underneath in order to avoid damaging the desk.  You could use pointy scissors for this step, but a razor-type blade makes the job much easier. (Obviously, an adult should do this step.)

The finished template (with my “helpers” in the background):

Now you are ready to encode.  Place the template over a fresh sheet of paper.  Mark around the corners with a pen to make it easier for the decoding person to line it up.  Write your message (one letter goes in each square).

Remove the template and fill in all the open squares with random letters.  Can you figure out what this says?

Here is the solution:

(“The girls are hiding the treehouse.” – OK, so it was supposed to say “The girls are hiding in the treehouse but we forgot the “in.” But coded messages are supposed to be brief, right?)


  • You can make your template as big or as small as you like.
  • For ease of communication between spies, you really ought to make a duplicate template so both sender and recipient have their own.  Just place the first template over a new piece of paper, trace the locations of the squares, laminate, and cut out.
  • I numbered the corners of the template 1 – 8 because you could use both sides and all four orientations to create different messages, or even one long one.
  • Traditionally, the template was placed over an ordinary letter (see the example here) but it can be very challenging to come up with a natural sounding message built around the coded text.  I gave it a try though and it was a fun mental exercise:

(“Arrival at six PM.”)

3 Responses to “Secret Codes: The Cardan Grille (“Messages” Unplugged Project)”

  1. aly in va says:

    I can see my girls really having fun with this. I’d expect to see lots of “randsom pirate notes”….thanks for sharing.

  2. Very cool! I’m having enough trouble trying to figure out the “backwards” and “phonetic-spelled” codes at my house…

  3. […] Unplug Your Kids has the secret code!  Laminate your decoder key and let the espionage begin! […]

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