The en prison rule emerged in France around 1700, probably in Lyon, though records are sparse. It was a player protection mechanism, designed to reduce the house edge on even-money bets. You would see it in European casinos by the late 18th century, and it persists today on certain wheels, though American casinos abandoned it entirely.
Origins and Early Practice
When roulette arrived in France, there were already rules for even-money bets on red/black, high/low, and odd/even. The en prison rule allowed players to avoid losing even-money bets when zero appeared. Instead of losing the bet immediately, the stake would be "imprisoned" for one more spin.
In the original version, if you had bet on black and zero came up, your black bet would remain on the table for the next spin. If the next spin hit black, you got your original stake back, unharmed. If the next spin hit red or zero again, you lost. This was a concession to players who felt robbed by the zero.
The rule shows up explicitly in gaming records from Monte Carlo in the 1860s. Francois Blanc, who pioneered the Monte Carlo casino, used it as a marketing advantage. Players would travel to Monaco partly because the en prison rule made their expected loss smaller than in other European locations.
The Mathematics
On a single-zero wheel, the house edge on even-money bets is normally 2.7%. The en prison rule reduces that to 1.35%. This is significant. Over 1,000 spins betting on red, the difference between losing 27 units and losing 13.5 units is real money.
Later, some European casinos introduced la partage, which is similar but not identical. La partage means you lose half your bet when zero hits, rather than playing for the full stakes on the next spin. Mathematically, la partage and en prison produce the same house edge.
Application Today
You will not see the en prison rule in American casinos. The double zero on US roulette wheels creates a 5.26% house edge, and casinos there have no incentive to reduce it. American players accept this because most have never seen anything better.
European casinos still offer en prison, though not on all wheels. If you travel to France or Monte Carlo, you'll see it on European-wheel tables, usually marked clearly. You may see wheels offering both double-zero American-style games and single-zero en prison games.
Online casinos with European licensing often include en prison rules on their single-zero wheels. This makes them mathematically preferable to American wheels, all else equal.
Why Players Misunderstand It
The en prison rule feels like an advantage because it feels like you're given a second chance. This is psychologically powerful. You didn't lose; you're in prison, which sounds temporary. When the next spin hits your color, you feel relief rather than luck.
But the rule is not an advantage. It is a reduction of a disadvantage. You still face a house edge. The en prison rule just makes that edge smaller. Some players use this to justify longer sessions. They reason that a 1.35% edge is small enough to be beaten through luck. This is incorrect. The edge compounds with every spin.
Historical Significance
The en prison rule matters in gambling history because it shows an early example of market competition in gaming. Casinos offered it because casinos in other locations were offering it. Players, given the choice, preferred tables where they could lose less to the house. This preference shaped the evolution of roulette rules.
Monte Carlo's reputation partly rested on offering the en prison rule on superior wheels. When that single advantage was known, it drew wealthy players. The mathematics were favorable compared to other European options.
Today, the rule persists because it's become part of European gaming tradition. Removing it would alienate players. But the rule serves the same function it did in 1700: it makes players feel more fairly treated by the house. Whether that feeling is justified depends on whether you were comparing yourself to American wheels or to other bets at the same wheel.
