Cardan Grille - Construction

Construction

A Cardan grille is made from a sheet of fairly rigid paper or parchment, or from thin metal. The paper is ruled to represent lines of handwriting and rectangular areas are cut out at arbitrary intervals between these lines.

An encipherer places the grille on a sheet of paper and writes his message in the rectangular apertures, some of which might allow a single letter, a syllable, or a whole word. Then, removing the grille, the fragments are filled out to create a note or letter that disguises the true message. Cardano suggested drafting the text three times in order to smooth any irregularities that might indicate the hidden words.

The recipient of the message must possess an identical grille. Copies of grilles are cut from an original template, but many different patterns could be made for one-to-one correspondence.

The grille can be placed in four positions – face up and face down, upright and reversed – which increases the number of possible cell positions fourfold.

In practice it can be difficult to construct an innocent message around a hidden text. Stilted language draws attention to itself and the purpose of the Cardan grille is to create a message “without suspicion” in the words of Francis Bacon. But the task was easier for Cardano, because 16th century spelling was not standardised and left much room for contractions and adornments of penmanship.