When a zero turns up, the player has two options: Reclaim half the bet and lose the other half. Leave the bet (en prison = in prison) for the next spin of the roulette wheel for an all or nothing gamble. If the subsequent spin is again zero, or does not match the imprisoned bet, then the whole bet is lost.

Zero is neither red nor black, high (passe) nor low (manque) in roulette, nor is it counted as either even or odd. Mathematically, zero is even, but if roulette treated it as such, then pair would be marginally an odds-on bet paying off at even money, which would be a loss to the house.

Martingale is definitely one of the most widely known and most successful roulette strategies (for some it is the best roulette strategy), and it is very simple to apply. It can be used only for outside bets, those that provide almost 50% chance of winning, and excluding the 0.