I wrote of Nicolaas Govert de Bruijn's passing back in February, but only gave a brief idea of the powerful magic his work made possible.
Grey Matters favorite James Grime has just released an excellent video that will give you an idea of how mystifying the use of de Bruijn sequences can be.
In the following video, James Grime will describe the trick, and then explain the workings behind it:
Quick note: You should use 3 decks to make a 48-card deck, not 4 as stated in the video. 48 cards is close enough to a regular 52-card deck that very few people will notice there are missing cards.
If you think about it, this works much in the same way as a trick with marked cards, but the people themselves unwittingly become the markings!
While you could practice and perform the method just as in the video, this wouldn't be Grey Matters if I couldn't improve the method with some memory techniques.
First, the code you're going to get from your audience is effectively a binary code. We'll assume that you have people take cards from your left to your right. We'll also assume that you're thinking of black as 0 and red as 1.
For example, if the 2nd and 4th spectators from your left are the only ones to respond to the red card question, your code would be 0101.
What you need atthis point, is an easy way to memorize any group of 4 binary digits. Over in the Mentat Wiki, their page on Binary Number Systems has a section that works perfectly, called the Nybble (4-Bit) Method.
Once you know all the names for each binary group of 4, you'll need to be able to recall cards. In the Mental Gym, I teach Bob Farmer's ingenious playing card mnemonics. With the practice quiz, you can learn these mnemonics quicker than you think!
As with any memory technique, we now need to link the information we have to the information we need.
First, you need to memorize the complete sequence of cards, using the card mnemonics in the following way:
Note that the first 3 are repeated at the end, so as to cover all possibilities.
To remember this, use the Link System approach of creating wild and crazy connection. Picture an ace coming out of your mouth (to remember it as the first card on the list), which then becomes a fist that starts hitting a narc who turns out to be someone or somthing named Cosmo, and so on. Make sure you memorize the entire story, including the repetition of -ace-fist-narc at the end.
Below is a chart of all the binary combinations, the code names given in the Nybble System, the mnemonic of the 1st card in that sequence, and the cards:
Assuming you know your binary mnemonics, your card mnemonics, and the sequence, all you have to do now is focus on the 2 center columns. To remember what cards are coded by 1100, you should easily recognize this as Minor.
Minor should be linked in your mind to seahorse. Perhaps you imagine someone under 18 (a minor) riding a seahorse around under the sea. Alternatively, you might picture a miner with a hard hat riding a seahorse. One way or another, the binary code of Minor should bring seahorse instantly to mind.
Once you have the first card in the sequence (seahorse, or 7H in our Minor example), you use your knowledge of the full sequence to recall the other 3 cards. The next 3 words in the sequence are jade, tombstone, and fork, so the full sequence is 7H, JD, 2S, and 4C.
If you're only planning to do this routine occasionally, the way James Grime teaches it in the video is fine. If you're going to use it regularly, however, the approach I've just detailed will serve you better over the long run.
For those who would like to learn more about this type of magic, you'll probably want to read one or more of Leo Boudreau's books, all of which take advantage of binary codes and de Bruijn sequences.
If you're a member of The Magic Cafe with 50 or more posts, you can find a good selection of Leo Boudreau's tricks for free at my Free Tricks from Leo Boudreau post.