Every time a player loses a round, the player will raise their bets by one unit.  A unit is the amount of money used for placing a bet.  The player will decide what the unit is.  For example, one unit could equal $10.  Therefore, if the player had bet one unit on the first round, and they lost the round, they will then bet $20 on the next round.

Every time a player wins a round, the player will lower their bet by only one unit. The D'Alembert Betting System can be played as an insurance system.  This is done by putting aside the units that have just been taken off after a win, and keeping them aside!  When you decreas the size of your bet when you win, especially if it follows a higher bet beforehand, you are keeping aside some of the profit from your winning round.

In order to play the D'Alembert betting system, the player needs a fairly large bankroll.  Without having a large bankroll, the losses made may not be able to be recouped and the player could have rather large losses.  This system helps to ensure (or should we say insure) a win after a losing streak.