En prison applies specifically to even-money bets when zero is the spin result. Rather than losing the full stake immediately, the bet is held on the layout and carried forward to the next spin. Whether the player recovers that stake or whether he loses it will depend entirely on the outcome of the next spin.
How does en prison hold the bet?
If zero appears on a table with en prison already active, then all even-money chips that are currently on the table are placed into a holding state. At crypto.games/roulette/bitcoin, this rule is applied to red, black, odd, even, high, and low bets. The chip does not move from its original position, but it is marked as imprisoned rather than active for the standard payout calculation.
On the next spin, three outcomes are possible.
- If the spin result matches the imprisoned bet, the player receives the original stake back with no profit. The chip is returned in full, but no winnings are added.
- If the spin result is the opposite of the imprisoned bet, the stake is lost and removed from the layout.
- If zero appears again on the second spin, the rule is applied again on some tables, imprisoning the bet for a third spin. Other tables treat a second consecutive zero as an immediate loss. Providers specify which interpretation applies.
What does en prison do to the house edge?
Without en prison, the house edge on even-money bets on a single-zero wheel is 2.70 per cent. Zero produces a full loss 1 in every 37 spins on average. En prison converts those full losses into a partial risk rather than a certain loss. When zero appears, the imprisoned bet faces approximately a 48.65 per cent chance of being returned intact and approximately a 48.65 per cent chance of being lost on the following spin, with approximately a 2.70 per cent chance of hitting zero again. Across many zero events, roughly half the imprisoned stakes are recovered, and half are lost. This approximately halves the cost of zero outcomes, which reduces the overall house edge on even-money bets from 2.70 per cent to approximately 1.35 per cent.
En prison compared to la partage
En prison and la partage both respond to the zero result on even-money bets, but they resolve differently. La partage returns half the stake immediately when zero appears, ending the round for that chip. En prison holds the full stake and resolves it on the next spin rather than splitting it immediately.
Using both rules will reduce the house edge from 2.70 per cent to around 1.35 per cent over a long period of time. The imprisoned stake is either returned in full or lost in full, rather than being guaranteed as a 50 per cent partial return as with the share. Whatever happens, partition ensures a half-return. En prison delays this calculation and introduces the likelihood of a full recovery or loss.
