A betting progression. It is a system where the bettor raises the bet one unit after each loss and lowers the bet one unit after each win. So let’s say the player starts out by betting 1 unit. If he wins, he continues to bet one unit. If he loses, he cancels out the 1 and moves to the 2 and adds one unit to the last number, now having a series of 2, 3, 4, 5. At any point in the series where the player wins his bet, he reduces his bet by one unit. If he wins enough bets to return to a one unit bet, he starts over. If he loses during the series, he cancels out the last number he played and adds another number to the series.