(apologies to John Bannon)
All right, how to make use of this principle. Let's say that we've just done a routine starting with "Mode To Order" (July 2000 Linking Ring), using the thirteen Diamonds, then continued with Curry's classic "A Swindle of Sorts" (Paul Curry Presents, 1974). To finish, have one spectator mix the cards, then take off a small packet. A second spectator notes the top card of the cards remaining. Take the cards from him and casually shuffle them by simply running the top card to the bottom, slip-shuffling several times, then running it back to the top. Have the first spectator count his packet—let's say he has four cards. Hand your cards back to the spectator that noted a card and ask him to transfer four cards from the top to the bottom and turn over the top card. It's not his card. He repeats this seven more times, each time getting a card that's not his card. Have him spread the cards so he can see only one remains face-down. It's his card.
This works because if you cut off any packet from thirteen cards, both packets will now be co-prime: 2-11, 3-10, 4-9, 5-8, 6-7. Note that this holds true for any packet with a prime number of cards.
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