The en prison rule
is for the game of roulette
and is more frequently used outside the US. The rule applies to the outside even money wagers only. The en prison rule is invoked when the spin of the wheel results in a zero or double zero. The rule varies in the way it handles a wager "en prison" when a zero appears on the next spin
In some casinos the wager is lost and in others the wager stays "en prison". If you have a $10 bet on red and zero shows up the dealer gives you the option of losing half your bet or going into prison. If you choose to go to prison the dealer will put a marker on top of your bet that reads "En Prison." On the next spin this happens:
1.If zero or double zero show you lose the $10, or otherwise, depending on the rules of the casino, your wager stays en prison.
2.If Black shows and you have red you lose the $10
3.If Red shows, the croupier removes the "En Prison" marker and you keep the $10.
The same applies for other even money wagers. A bet on even would go "en prison" on a zero, for example, and then would lose if the next spin was an odd.