En Prison
“In prison”: an even-money betbetEven-money betRed/black, odd/even, high/low. 18 numbers, pays 1:1.Read full → caught by zero gets one more spin instead of losing.
Your chip is held; if the next spin wins, you get the stake back (no winnings). Mathematically close to la partage: about 1.35% edge.
Sequence
ZerowheelZeroThe single green pocket that no even-money, dozen or column bet covers; the source of the house edge. Also: single zero, 0.Read full → hits. Your $10 redbetRedEven-money bet on the 18 red numbers. French: rouge.Read full → chipmoneyChipA token representing a fixed amount, placed on the layout to make a bet. Also: check.Read full → is not swept; the dealer places a marker on it, or slides it onto the line of the box, to show it is imprisoned. The next spin is played. Red: the chip is released and returned to you, $10, with no winnings. BlackbetBlackEven-money bet on the 18 black numbers. French: noir.Read full →: the chip is lost. Zero again: house rules vary. Some casinos release half (effectively la partageruleLa partageFrench rule: when zero hits, half of an even-money stake is returned. Halves the house edge on those bets to 1.35%.Read full →), some keep it imprisoned for another spin, a few lose it outright.
Zero hits. Your $10 on red is marked “en prison”.
Chip released. $10 back, no winnings. Net for two spins: $0.
Chip lost. Net: −$10.
Depends on the house: stays imprisoned, half returned, or lost.
The arithmetic
On the spin after zero the imprisoned chip wins back its stake with probabilitymathProbabilityChance of an outcome, from 0 to 1 or 0% to 100%. One number: 1/37 = 2.70%.Read full → 18/37 and is lost with probability 18/37 (ignoring the second-zero case). Its expected valuemathExpected valueThe average result per bet over the long run. −1/37 of the stake for every standard European bet.Read full → is therefore about half the stake, the same as la partage returning half immediately. If a second zero loses the chip, en prisonruleEn prisonFrench rule: when zero hits, an even-money bet is held for the next spin instead of lost. Halves the house edge on those bets.Read full → is very slightly worse than la partage: 1.39% against 1.35%. If a second zero simply keeps the chip imprisoned for another spin, the two are identical. In the strict French rule a second zero sends the chip one level deeper, and it then needs two wins to come out: 1.37%. The difference is small enough to ignore when choosing a table.
Why the rule exists
En prison is the older of the two French rules and was a courtesy to regulars at 19th-century spa casinos. La partage is the modern simplification: same value, no bookkeeping, no argument about a chip left on the line. Monte Carlo tables historically let the player choose when zero hit; most tables today apply one rule automatically.
Interaction with your own rules
If you play a stop-lossstrategyStop-lossA pre-set amount of loss at which you end the session.Read full → or a fixed number of spins, an imprisoned chip is a bet you did not choose to make on the next spin. It is a small point, but it is the only situation in roulette where the table makes a decision for you.
Questions
No. It stays until the next spin settles it. Some houses let you take half instead, which is la partage by another route.
No. Even-money bets only, like la partage.
They are the same to within 0.04%. Take whichever the table offers.
Fun with Math
Try these yourself first. Each solution is folded; open it when you are ready.
Under en prisonruleEn prisonFrench rule: when zero hits, an even-money bet is held for the next spin instead of lost. Halves the house edge on those bets.Read full →, an even-money betbetEven-money betRed/black, odd/even, high/low. 18 numbers, pays 1:1.Read full → caught by zerowheelZeroThe single green pocket that no even-money, dozen or column bet covers; the source of the house edge. Also: single zero, 0.Read full → gets one more spin: a win returns the stake, anything else loses it, a second zero included. Work out the house edgemathHouse edgeThe share of each stake the casino keeps on average: 2.70% in European roulette.Read full →.
Show the solution
Answer 19/1,369, about 1.39%.
Take $1 on redbetRedEven-money bet on the 18 red numbers. French: rouge.Read full →. It wins outright with chance 18/37: +1.
Zero comes up with chance 1/37. Then the next spin returns the stake with chance 18/37 (net 0) and loses it otherwise. So the bet loses with chance 18/37 on the first spin, plus 1/37 × 19/37 after a zero: 666/1,369 + 19/1,369 = 685/1,369.
Expected valuemathExpected valueThe average result per bet over the long run. −1/37 of the stake for every standard European bet.Read full →: 666/1,369 − 685/1,369 = −19/1,369 ≈ −1.39%. La partageruleLa partageFrench rule: when zero hits, half of an even-money stake is returned. Halves the house edge on those bets to 1.35%.Read full → hands back half at once and costs 1/74 ≈ 1.35%: the second zero is the whole difference.
Comic relief
Freedom, but no winnings.
Sources
- Ethier, S. N. The Doctrine of Chances: Probabilistic Aspects of Gambling.
- Wykes, A. The Complete Illustrated Guide to Gambling.
- Wizard of Odds. “Roulette Basics.”