Visions, The Online Journal of the Art of Magic

Werner Miller is a retired teacher of mathematics, whose hobbies are recreational mathematics and magic. A magical inventor and writer, Werner has produced a prolific volume of work, mostly semi-automatic card tricks and other self-working miracles. He is the author of "E-Z Square 1 & 2", the "Enigmaths" series and several other books and ebooks on mathematical magic, besides more than 300 contributions to various magazines and web sites. You can find his tricks regulary in, e.g., the German magazine "Magische Welt" and the British "Mystery Magazine", as well as here at Visions.

If you are performing for a chess player (or if you, at least, can be sure the spectator knows how to move a knight on a chess board: one square vertically and two squares horizontally, or two squares vertically and one square horizontally) try this:

Get ten blank file cards. Let the spectator select one, have it signed and placed back on top, signed side down.

Mix the cards by giving them an Under/Down Deal, dealing the "down" cards into a 3x3 matrix, row by row and from left to right. Unknown to the spectator, the signed card will be dealt fifth and will end up in the center cell of the matrix.

Now hand the spectator a real chess knight piece (or any other small object) and ask the spectator to put it on any card of the matrix.

If the spectator places the piece on the center card turn over this card and show the signature: The spectator has successfully located his own selection.

If the spectator did not place the piece on the center card discard this card and request the spectator to move on to any other card by making a knight's move in any direction (in fact, there are only two possibilities: to go either clockwise or counter-clockwise). The card the piece ends up is to be eliminated, too. Another knight's move is to be done and so on until - after the sixth move - all cards but the center one have been taken away and there are no more cards the knight can move to. Turn over all the discarded cards and show that they are blank. Have the spectator turn over the remaining card revealing the signature.

Postscript
The fact that a chess knight can visit the eight border cells of a 3x3 matrix in a closed loop forms also the basis of Henry Dudeney's famous puzzle "The Four Frogs". See, e.g., http://headinside.blogspot.com/2011/04/knight-shift.html