A roulette wheel has $38$ slots, numbered $0,00,$ and $1$ to $36.$ The slots $0$ and $00$ are colored green, $18$ of the others are red, and $18$ are black. The dealer spins the wheel and, at the same time, rolls a small ball along the wheel in the opposite direction. The wheel is carefully balanced so that the ball is equally likely to land in any slot when the wheel slows.

Gamblers can bet on various combinations of numbers and colors.