Named for its inventor, Girolamo Cardano (1501–1576), the Grille system works in the following way: Each recipient has a piece of paper or cardboard with holes cut in it (the grille). When the grille is placed over an innocent-looking message, the holes line up with specific letters in the message, revealing the hidden message within.
Intercepting these messages becomes very difficult at this point because the larger message, which often takes up a page, completely blends the shorter, secret message into it. As with jargon code, this type of steganography requires imagination and good writing skills. Cardano's Grille is considered one of the safest ways to transmit a secret message.