The Martingale system is the most common betting system in roulette, and is also used in other casino games. The system is simple: you bet on a color, and if you lose, you double your bet.
If you lose again, you double your bet again. You keep doing this until you win, at which point you go back to betting the original amount.
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The idea behind the Martingale is that eventually you will win, and when you do, you will recoup all of your losses plus a profit equal to your original bet. The problem with the Martingale is that it can be very risky; if you have a long losing streak, you can quickly rack up a large amount of debt.
There are variations of the Martingale that can help to mitigate some of the risk; for example, instead of doubling your bet after a loss, you could increase it by a smaller amount. But at its core, the Martingale is a high-risk/high-reward betting system, and it’s not for everyone.