Cardan Grille

What is a Cardan Grille?

A Cardan grille (also called Cardano grille) is a cryptographic tool invented by Girolamo Cardano in 1550. It's a sheet of stiff material (like cardboard or metal) with holes cut in specific positions. When placed over a text, the holes reveal a hidden message while the rest of the text serves as camouflage.

How the Cardan Grille Works

Basic Grille

1. Create a grid pattern with holes in specific positions
2. Place the grille over the ciphertext
3. Read only the letters visible through the holes

Rotating Grille

A more sophisticated version uses a square grille that is rotated 90° four times:

1. Read letters through holes at 0°
2. Rotate 90° clockwise, read more letters
3. Rotate 90° again (180° total), read more
4. Rotate 90° again (270° total), read final letters

This allows hiding a message 4 times longer than the number of holes.

Cardan Grille in Geocaching

Cardan grilles are popular in geocaching puzzles because:

• Visual puzzles: The grille can be hidden in an image
• Physical interaction: Print-and-cut solutions are satisfying
• Steganography: Messages hide in plain sight
• Layered puzzles: Finding the grille pattern is part of the challenge

Creating a Cardan Grille

For a rotating grille to work, holes must be placed carefully:

• Each hole position, when rotated 3 times, must hit 4 different cells
• For an N×N grid, you can have at most N²/4 holes
• Common sizes are 6×6 (9 holes) or 8×8 (16 holes)

Recognizing Cardan Grille Puzzles

Look for these clues:

• Square blocks of text with seemingly random letters
• Images with regular hole patterns
• References to Cardano, grilles, or masks
• Text that doesn't make sense when read normally

History

Girolamo Cardano (1501-1576) was an Italian polymath who invented this cipher. The grille was used for centuries, including during the American Revolutionary War. Today it remains popular in puzzle games and escape rooms.