American Roulette with the En Prison rule is found in many online casinos. In real institutions, this method is also common, although not in all countries.
En Prison – a rule in force in some varieties roulettes.
It concerns bets on equal chances: red/black, large/small numbers and odd/even.
When zero is dropped, they do not immediately lose, but remain on the field is like "in prison" (hence the name). The dealer marks them with a marker. If in the next round the bet wins, the player gets it back without extra payments.
The Rule En Prison reduces the mathematical superiority of the casino in betting on equal chances in two times, to 1.35%.