En prison rule

This is also known as the surrender rule. Mostly applicable in European roulette; in an en prison rule, when the player makes an outside bet (even money, or red/black), and then the ball drops on 0, the player has the option to lose half the bet or retain the money at stake en prison, meaning leaving it on the table for the next spin. When a player opts to leave it, the chance or the edge of the house is considerably lowered in the next spin.