Rechenkönig (a.k.a. King of Numbers) is a collection of number-learning games using a base set of cards that are cleverly designed. One set of cards has a simple equation, like "2 + 1" or "9 -7". Another set has solutions, like "=3".
The cards have two special features. First, on one side graphics are shown with the numbers, while on the reverse the graphics are not shown. This allows the games to played at different levels based on the age of the children. Second, each card has a hole in it, and the placement of this hole aligns with its solution cards so that when the proper answers are placed together, a wooden "king of numbers" figure fits precisely into the combined hole; if the king doesn't fit, the answer is wrong. The wooden King piece doubles as a pencil holder, which is helpful since some of the games require scorekeeping on a piece of paper.
Using this card set, the rules describe seven different games of increasing difficulty. One of these is a solitaire game, the others are for 2, 3, or more players. Each is interesting and the variability of the play with the unique cards keeps the game interesting through multiple plays and useful for children.