Here is the description From Wallace Lee's book Math Miracles 1950 |

The Magician and Assistant have a secret code to communicate the hidden card.

The math behind this uses the Pigeon Hole Principle, Permutations and Modular Arithmetic

Historical Note: Fitch Cheney performed this trick in the 1930's and 1940's - before cell phones were invented. Not every home had a phone then, and some homes were still on party-lines.

Alternate Trick: John Northern Hilliard did a trick called "Twentieth-Century Telepathy. This was not presented as a card trick but as metal telepathy.

In a room full of people, he would ask one of them to name a card - "any card". When the person did, he would hand them a piece of paper; tell them to call the number on it, and ask for his assistant. The assistant would come to the phone, name the card, and hangup.

You can find more information in Karl Fulves book, More Self-Working Card Tricks. Chapter 3. (Should be on reserve in the library.)

- Original Article In Wallace Lee Book
- More Detailed Explanation Written by a Teacher. May be best for presenting a lecture on the material.
- Colm Mulcahy Article
- Another Colm Mulcahy Article from MAA
- Norman Do More Explanation
- n-card Version Generalizing for different numbers of cards
- The best card trick This one extends it to 124 cards and explains the connection to the Birkoff-von Neumann Theorem and Hall's Marriage Lemma.
- Using a Card Trick to Teach Discrete Math This ones goes into greater depth. Discusses permutations, combinations, the greedy algorithm,

Here, is Charlie's crib for the trick

Today, some people have replaced the assistant with a computer.
Some have programmed their cell phones as their assistant. Some have even programmed their TI graphing calculators. Larry Finley Using a computer program he wrote in "Just Basic" (don't feel obligated to watch the whole thing) | |

Entirely Different Trick Bill McCaffrey liked the idea of Phoning Someone So he came up with a trick called The Wizard Here is the writeup from John Scarne's book. |