Have someone choose a number from 1 to 16 and these amazing alder wood cards will quickly determine the number. The secrets of these long lost cards were found in a 1942 book by Maurice Kraitchik called Mathematical Recreations. They are also called window cards and have a fun magical edge to them. Each card is 1.3" x 4" x 1/8" thick alder wood with the numbers and text deeply laser engraved. Also look for our other more complex version of the cards on the website.