In the Martingale roulette system, each bet is doubled after a loss, and you go back to your starting bet after every win. You will win one unit for every win, as long as you have enough money and do not get to the house limit. Once you reach the house limit, then you will lose the house percentage because there is noplace else to go. There are many variations of Martingale roulette system, and it can be used with other methods. In this latter one you are betting only one bet on each shooter.
Some people add one or more units to each of the doubled bets.
Each bet is the first and last of these numbers. In this example, it is 1 and 3, which equals 4. If you win you cross the two number off and bet the next two that are on the outside. In this instance there would be only the single two unit bet. If the bet loses, then you add that one number to the end of the series.