The En Prison version of roulette gambling was introduced in the 19th century. En Prison is the European version of roulette without the double 0. This form of the game lowered the house’s edge further down from 2.7% to 1.35%. Because of this very low house edge, the game of roulette began to gain popularity which grew rapidly. In Europe, roulette games count for 50% of casino earnings. However, in the United States where many casinos only offer the double 0 version, roulette is only 5% of casino earnings.