En prison in roulette

The rule only wakes up when two conditions meet: the wheel has a single zero, and the bet in question pays even money. That means red or black, odd or even, and the low or high halves of the layout, 1-18 and 19-36. Straight-ups, splits, dozens and columns are untouched and lose on zero as they would anywhere else.

Once zero lands, the stake is marked as imprisoned, often with a marker on the line of the betting area or a flag on the digital layout. The player cannot pick it up or move it while it is held. On the next spin, a result matching the original bet releases the stake with no winnings attached, and any other result forfeits it.

Nothing is won in prison.

When zero comes up twice

Practice varies here, and it is the detail that decides the true edge. Some tables keep the bet imprisoned a second time, some treat a repeat zero as a straight loss, and some return half the stake at that point. The rule panel or help screen of the game is where that wording sits, and it is worth reading before quoting any percentage for a particular table, because two games both labelled French roulette can settle the same sequence differently.

On an ordinary single-zero wheel, an even-money bet loses on 19 of the 37 pockets and wins on 18, which leaves the house a margin of one pocket in 37, or about 2.70% of every euro staked. En prison changes only what happens on that single zero pocket.

Take a table where a second zero loses the imprisoned bet. Zero arrives one spin in 37, and from there the stake comes back with a probability of 18 in 37, so the house keeps it only 19 times in 37. Multiplying those odds out gives a long-run cost of 19 parts in 1,369, which is about 1.39% of the amount wagered.

Where a repeat zero keeps the bet in prison rather than killing it, the sum settles at one part in 74, or about 1.35%. That is the same figure la partage produces, and it is the number most often printed for both rules.

In plain money, someone staking €1,000 in total on even-money bets at a 1.39% table can expect to give back around €13.90 as a long-run average, against about €27 at a single-zero table with no zero rule. That is an average across a very large number of spins, never a forecast for one evening; a short session can swing far above or below it, and no run of results makes zero more or less likely on the following spin.

The two rules are confused constantly, partly because many sites describe them as identical. La partage settles the bet at once: when zero lands, half the even-money stake goes back to the player and the other half is lost. En prison delays the decision by a spin and gives the player a chance at the whole stake, at the risk of losing all of it.

Both cut the zero’s cost roughly in half, so the long-run gap between them is small. Under the common variant where a second zero loses, en prison is slightly dearer at about 1.39% against 1.35%, while the more generous repeat-zero settlements bring it level with la partage. The real difference is in variance: la partage hands back a certain half, and en prison turns that half into an all-or-nothing spin.

Some land-based casinos offer the player a choice between the two when zero lands, while online and live casino tables usually fix one rule in the game’s settings. A layout printed in French is no guarantee that either rule is switched on, so the help screen is the reliable source, not the table’s name.

European roulette without a zero rule keeps the full 2.70% on every bet.