Sympathetic alphabets were an early (around the 17th century) attempt at long-distance communication. Following the principles of sympathetic magic, it was believed that if two people exchanged strips of flesh through surgery and implantation there would exist a certain resonance between them. To properly communicate, the alphabet was tattooed upon the transplanted flesh for both. When they wished to communicate to each other, one merely pricked letters with a magnetic needle, and the pain would show up in the letters of the other person's arm.

An interesting approach at a human telegraph, but there's no information on whether the idea was ever successful. Considering the less than sterile environment of medieval surgery, I doubt they ever got past the flesh transplants before the experimenters died.