A mysterious man called the Shadow was a popular serial hero in the 1930s.
The story “Chain of the Code” contained a so-called “directional code”. In addition to the characters for the 26 letters of the alphabet and a space, there were four other symbols 1 – 4. The lines represent directional arrows and indicate in which direction the leaf with the message should be held:
- Symbol 1: The sheet is held normally
- Symbol 2: The sheet is turned 90 degrees to the right
- Symbol 3: The sheet is rotated 90 degrees to the left
- Symbol 4: The sheet is rotated 180 degrees
The extra symbols can be written at any time, making frequency analysis more difficult.