(Roulette continued)

En Prison

     

The En Prison rule is a special rule used in some casinos in the world. As of this writing, I do not know of any casinos in the United States that use this rule. The En Prison rule applies only to the even money wagers. The way the rule is used varies slightly, but I believe the following is the most common implementation:

If a zero comes up, bettors are allowed to either (1) lose half of their even money wager or (2) let it ride for another spin. If a green zero appears on the second spin, the wager stays "en prison", otherwise the wager is lost if the wrong numbers/color comes up and pushed if the players numbers/color comes up. The effect on the house edge for either of the two options is the same. In single zero roulette it works out as follows.

• probability of even money wager win = 18/37
• probability of even money wager loss = 18/37
• probability of zero invoking en prison = 1/37
• probability of zero coming followed by a losing decision on the next spin (1/37 * 18/36)

Therefore :
(1) Lose Half Wager = 18/37 * 1 - (18/37 * 1 + 1/37 * .5) = -.0135 = 1.35%
(2) Spin one more = 18/37 * 1 - (18/37 * 1 + (1/37 * 18/36) * 1) = -.0135 = 1.35%