Caesar cipher wheel

The outer ring holds the letters of your message and the inner disc holds the cipher letters. Turn the inner disc and every pair lines up at once. With the wheel at shift 3, A sits over D, B over E and so on round to Z over C. To encode, find each letter on the outside and write down the one beneath it. To decode, go the other way, from the inner disc out.

The box at the top of the wheel marks the A, so the letter under it tells you the shift. That's the key to share with whoever will read the message. The numbers 0 to 25 under the outer letters give each letter's place in the alphabet, which helps when the shift is given as a number, or when you're checking the sum by hand.

Print the sheet at full size, with no scaling, and the big disc comes out 14 cm across and the small one 10 cm. Thin card works better than paper if your printer takes it. Cut round both circles, lay the small disc on the big one with the centre marks matching, and push a split pin through both. Open the legs of the pin flat at the back, loosely enough that the small disc still turns.

The blank version leaves the inner disc empty. Write a scrambled alphabet in its 26 spaces and the wheel makes a different cipher, one where every letter moves by a different amount. A class can make a set where each pair of students has its own inner disc, so messages from one pair can't be read with another pair's wheel.

The cipher disc was described by Leon Battista Alberti, an Italian architect and writer, around 1470. His version was two copper plates, a large fixed one and a smaller one on top that could turn, each with an alphabet round its edge. Alberti's idea was to turn the disc now and then during a message, so that one setting didn't cover the whole text.

Discs like this were still in use in the American Civil War, when the Union army had a cipher disc 3.75 inches across. From the 1930s, cheap versions were sent out as radio show prizes and sold as toys, often called secret decoders.