This is a system for determining the amount to bet at roulette. It is also known as "Cross-out", or "Cancellation".

In the simplest form, you write down some numbers. For example: 1 2 3
Each bet is the first and last of these numbers. In this case it is 1 and 3, which is 4. If you win, you cross out the two numbers and bet the next two that are on the outside. In this instance there would be only a single two unit bet. If that wins, the series is completed, and the winnings are the total of all the numbers (6 in this case). If the bet looses, then you add that number to the end of the series. Thus, the first bet would be 4. If this looses, then 4 is added to the series:
1 2 3 4
The next bet would then be 1 plus 4, which is 5.