The principle of the ball is simple. It involves throwing a red rubber ball in a bowl, itself embedded in a gaming table. The bottom of the bowl has a bulge around which nine holes are distributed. A bet table is drawn on the table, and the numbers from 1 to 9 are entered. Once the “games made”, that is to say when the money or chips have been wagered, the dealer who has thrown the ball announces the number corresponding to the hollow in which it stopped. Those who played the right number receive 8 times their bet. With a bet of 100 €, it is possible to win 8 × 100 = 800 €. However, the exit probability of each number is, in fact, equal to 1/9. The expected value is 800 × 1/9 = 88.88 €, a lower value of 11.11 € at stake. This difference, which makes the game unfair, represents the levy of the casino. For there to be equity, it would be necessary that one pays 9 times and not 8 times the bet in case of exit of the number played.