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Rotating grille

A six by six card with nine windows that turns four times and fills every square. Printable template included.

about 35 minutes the hardest one here makes a working machine

What you need

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

How to build it

Cut out the nine windowsAn adult holds the knife. Mark the dot corner clearly on both sides of the card.
Lay it on paper, dot at the top leftWrite the first nine letters of your message, one in each window.
Turn the card a quarter turnDot now at the top right. Nine new windows appear over nine empty squares.
Write the next nineThen turn again, and again, until you have gone all the way round.
Lift the card offThirty-six letters, no gaps, no order anybody can see.
Fill any spare squares with nullsIf your message is short, put in letters that mean nothing.
The windows have to be chosen so that four turns cover every square exactly once. Cut only the dashed ones on this template, or it will not work.

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 nine dashed squares. the dot marks the top corner
The bar above should measure exactly 10 centimetres. If it does not, your printer has shrunk the page.

Now send something

Hide THE PARCEL IS IN THE SHED BEHIND THE OLD GREEN DOOR, which is exactly thirty-six letters if you drop the spaces.

Where it comes from

Europe, from the 1700s
The card that turns four times

The improvement on Cardano: cut the windows so that turning the card four times covers every square exactly once. Nothing is wasted and nothing is left blank. Jules Verne liked it enough to build a novel around one, and armies used versions of it into the twentieth century.

Why it works

Turning the card is what makes it fill perfectly. Choose the windows badly and you leave holes or write twice in the same square, which is why the template matters more here than on any other build.

When it goes wrong

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

Some squares get written twiceA window is in the wrong place. Only cut the marked squares: they are chosen so four turns cover the grid exactly once.
There are gaps at the endYour message is short. Fill the spare squares with nulls, which are letters that mean nothing.

Where to go next

Route with nullsBack to Make

Read more elsewhere

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

Grilles