En prison (French for “in prison”) is a roulette rule that protects even-money bets when the ball lands on zero. Instead of losing straight away, a bet on red or black, odd or even, or high or low is “imprisoned” for the next spin. If that spin wins, the stake is released back to the player without winnings; if it loses, the stake is gone. While a bet is imprisoned, its amount and type cannot be changed.
The rule is not part of every game. It is offered on some single-zero tables, most often in French roulette, and some casinos use the similar la partage rule, which returns half the stake immediately. Either way, the house edge on even-money bets drops from 2.70% to about 1.35%, which makes these tables the best value for outside-bet players.
Make it visible
Because it halves the edge on even-money bets, en prison is a real selling point. Mention it in the game information panel and make French roulette easy to find in the lobby.