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Cardano grille

Cut a card full of windows so a dull letter about the weather carries a secret. Printable template and step by step.

about 25 minutes needs a craft knife and an adult makes a working machine

What you need

The printed template
Card
A craft knife and a cutting mat
A ruler
An adult for the cutting

How to build it

Print two copiesOne to cut, one for your friend. They must be identical or it will not work.
Stick to cardGlue the template flat onto card and let it dry, or the windows will tear.
Cut out the seven dashed squaresAn adult holds the knife. Cut against the ruler and take your time.
Write the secret firstLay the grille on paper, and write one word of your message in each window.
Lift the grille offNow fill in all the space around those words with an ordinary, boring letter.
Send the letter onlyYour friend lays their grille on top, and only your words show through.
Mark a corner of both grilles with a dot. Laid down the wrong way round, the windows land in the wrong places and the message is gibberish.

The template

Print this at 100 per cent, not fit to page, or the sizes will be wrong. The ruler line below tells you whether it printed correctly.

10 cmcut out the seven dashed squares
The bar above should measure exactly 10 centimetres. If it does not, your printer has shrunk the page.

Now send something

Hide GO TO THE OLD OAK AT SIX. The hardest part is writing a dull enough letter around it, which is exactly the skill.

Where it comes from

Italy, the 1550s
A card full of windows

Girolamo Cardano was a physician, a mathematician and a compulsive gambler who wrote the first serious book on probability, largely so he could win. His grille hides a message inside an ordinary letter. Four hundred years later, agents in occupied Europe were still cutting windows in card for the same reason: a cipher announces itself, and a dull letter about the weather does not.

Why it works

Nothing is enciphered at all. The letter is real, the words are real, and there is no key to steal. The secret is which words matter, and that lives in a piece of card.

When it goes wrong

It usually does, the first time. Almost always one of these.

The words come out in the wrong orderThe grille is upside down or back to front. Mark one corner with a dot on both copies.
The letter reads oddlyThat is the hard part, and it is the skill. Write the dull letter last, around the words you already placed.
The windows tearGlue the template to card and let it dry flat before cutting.

Where to go next

Hidden in Plain SightBack to Make

Read more elsewhere

Longer, harder and written for grown-ups. Worth a look when this page runs out.

Grilles