On the figure below you can find an overview of adjacent numbers on the wheel of the French Roulette. The wheel has got only one zero, unlike the American Roulette with a double zero.
The bets on the adjacent numbers can be used if you believe that the ball tends to fall more often into several parts of the Roulette wheel. That might well be true in a longer observation despite the fact that the wheels are manufactured with very high accuracy.
Figure – French Roulette Voisins