The Impact of the “En Prison” Rule in French Roulette

Why does the number zero feel so punishing in roulette—and why does French roulette treat it differently?

The answer lies in the En Prison rule, a distinctive feature that changes how losses are handled when zero appears on a single-zero wheel. While it does not alter the randomness of outcomes or allow players to predict results, En Prison meaningfully affects volatility, expected loss, and session pacing, particularly on even-money bets.

This article explains the En Prison rule from an educational perspective, focusing on probability, structure, and gameplay impact rather than advantage-seeking.

What this guide covers:

• What the En Prison rule is and when it applies
• How it works step by step
• Its effect on house edge and expected value
• How it compares with La Partage
• The impact on volatility and bankroll behaviour
• Differences across French, European, and American roulette
• How the rule appears in live and online roulette formats

Disclaimer: This content is for informational purposes only. Roulette outcomes are random and independent. No rule or system can influence results. Always play on licensed online casino platforms and gamble responsibly. 18+. For support, visit BeGambleAware.org.

What Is the En Prison Rule?

The En Prison rule (French for “in prison”) is an optional rule most commonly associated with French roulette, and occasionally offered on some European roulette tables. It applies only to even-money bets: Red/Black, Odd/Even, and High/Low.

When the Rule Applies

• Only on single-zero wheels
• Only on even-money bets
• Triggered when the ball lands on zero

It is never used on American (double-zero) roulette table games.

How En Prison Works — Step by Step

1. You place an even-money bet (e.g. Red).
2. The ball lands on zero (0).
3. Instead of losing immediately, your bet is “imprisoned” on the table.
4. On the next spin:
• If your bet wins → your original stake is returned (no profit).
• If it loses → the stake is forfeited.

Crucially, even if the second spin wins, you do not receive winnings—only your original bet back.

En Prison vs La Partage — Same Maths, Different Experience

French roulette is also known for the La Partage rule, which often leads to confusion between the two.

Mathematical Equivalence

From a probability standpoint, En Prison and La Partage produce the same expected value on even-money bets. Both reduce the standard European roulette house edge from 2.70% to approximately 1.35%.

The difference lies not in mathematics, but in how losses are resolved.

Behavioural Difference

• La Partage: Half your bet is immediately returned when zero appears.
• En Prison: The full bet is held over for one additional spin.

Although the expected loss is the same over time, En Prison feels more interactive because it delays the outcome rather than partially settling it instantly.

Probability and House Edge Explained Simply

On a standard single-zero roulette wheel, the probability of landing on zero is 1 in 37.

Without any special rule:

• An even-money bet loses entirely when zero appears.
• This produces a 2.70% house edge.

With En Prison:

• Half of all zero outcomes are statistically expected to be recovered on the next spin.
• This effectively halves the long-term loss rate.

In practical terms, En Prison reduces the expected loss on even-money bets to around 1.35%, identical to La Partage. For a deeper explanation of how this works across roulette variants, see this guide on roulette house edge.

What the Rule Does Not Change

• Win probability per spin
• Independence of outcomes
• Long-term negative expectation

En Prison softens losses; it does not create a player advantage.

Volatility and Bankroll Impact

Where En Prison has the most noticeable effect is volatility—how sharply wins and losses fluctuate during a session.

Loss Smoothing Effect

Because zero no longer causes an immediate full loss on even-money bets, bankroll drawdowns tend to be less abrupt. This often results in:

• Longer sessions at the same stake size
• Fewer rapid swings in balance
• A calmer pace of play

Why This Matters for Conservative Play

For players who prefer:

• Flat betting
• Fixed session budgets
• Slower, more deliberate gameplay

En Prison can make the experience feel more controlled, even though the mathematical expectation remains unchanged.

Interaction With Betting Systems (Descriptive Only)

Some players discuss En Prison in relation to common betting systems. It’s important to view this descriptively, not as an endorsement.

Flat Betting

With flat bets, En Prison simply reduces the frequency of full losses caused by zero. There is no compounding risk and no change to expected outcomes.

Progression Systems (e.g. Martingale)

En Prison can delay escalation because zero does not immediately count as a full loss. This may:

• Slow the pace of doubling
• Extend bankroll longevity in the short term

However, it does not remove the core risks of progression systems, including long losing streaks and table limits. For general context only, see this overview of roulette strategy concepts.

Comparing Roulette Variants

The En Prison rule only exists where the wheel structure allows it to function mathematically.

Variant	Zeros	En Prison Available	Effective House Edge (Even Bets)
French Roulette	1 (0)	Often	~1.35%
European Roulette	1 (0)	Optional	2.70% → ~1.35%
American Roulette	0,00	No	5.26%

This is why French roulette is often considered the most player-friendly version in terms of volatility, though not predictability.

En Prison in Live and Online Roulette

Modern online roulette platforms implement En Prison consistently across formats.

Live Dealer French Roulette

In a live dealer session, imprisoned bets are clearly marked on the table and explained verbally by the croupier, preserving the traditional pacing and etiquette.

RNG French Roulette

In RNG-based games, the rule is handled automatically by the software, with visual indicators showing when a bet is held over.

In both cases, probabilities remain identical—the difference is purely experiential.

Responsible Play Perspective

While En Prison reduces volatility, it does not reduce risk.

Longer sessions can sometimes encourage extended play, which makes budget limits and time awareness essential. Understanding expected loss helps players make informed decisions and maintain control.

Resources and tools for safer gambling are available via responsible gaming guidance in desktop or mobile app format.

Conclusion — What the En Prison Rule Really Does

The En Prison rule does not help players win more often, but it does change how losses unfold when zero appears.

Key takeaways:

• En Prison applies only to even-money bets on single-zero wheels
• It reduces effective house edge to ~1.35%, matching La Partage
• It smooths volatility rather than altering probabilities
• Its value lies in pacing and bankroll control, not advantage

For players exploring structured roulette experiences on licensed platforms, French roulette with En Prison remains one of the most balanced formats available at Prime Casino.

18+ | Play Responsibly