What is En prison?
En prison is a roulette rule under which an even-money bet is held, or 'imprisoned', for the next spin when the ball lands on zero, instead of being lost.
If the next spin wins, the original stake is returned without profit. If it loses, the stake is lost. Some casinos allow repeated imprisonment if zero appears again. The effect on the house edge is about the same as la partage: roughly 1.35% on even-money bets.
Many casinos let the player choose between en prison and taking half the stake back, which is the la partage outcome. Mathematically the two are nearly identical.
The rule applies only to even-money bets and is found mainly on French roulette tables in continental Europe and in some online versions. A player whose bet is imprisoned cannot usually withdraw it before the next spin. Where repeated zeros can keep a bet in prison for several spins, the casino's exact procedure is set out in its house rules, and these details vary between venues and online versions.