Applied to even-money bets. If the ball lands on zero, the bet is imprisoned for another spin. If the ball lands on zero during the next spin, the player loses the bet. This rule reduces the house edge by 50% on even-money bets.
The en prison rule (or surrender rule) is offered in some European roulette games. If you have made an outside evens-money bet (such as red/black) and the ball lands on 0, you may choose to lose half of your stake or keep your bet en prison (that is, leave it in place for the next spin). By leaving it “en prison”, you lower the house edge for your next spin.
When the outcome is zero, some casinos will allow the players to either take back half his bet or leave the bet (in prison) for another spin. In the second case, if the following spin the outcome is again zero, then the whole bet is lost. This rule applies to even money bets only.