It is a fundamental principle of gambling games that the greater the risk the player takes the more should be his expected return. Or stated conversely, if a player wants a high return on his bet then he has to take a greater risk. A greater risk is characterized by a lower chance of the bet winning. In roulette the fewer numbers the bet covers, the less is the likelihood of the bet winning and hence greater the risk. Therefore the bets that cover fewer numbers offer higher payouts and bets that cover more numbers offer lower payouts.
The least risky bets in roulette are the even money bets. There are three such bets. Each bet divides the 36 numbers into two groups in different ways. One division is even numbers versus odd numbers. Another division is the first eighteen numbers (that is 1 to 18) versus the last eighteen numbers (that is 19 to 36). The third division is red numbers versus black numbers. Half the numbers on the wheel are red and half are back. This is the most popular even money bet. These bets win more frequently than the other bets. But they have the lowest payouts at 1 to 1. $1 wagered on these bets will win only $1. Players who get comfort in winning more frequently even if they win small amount should go for these bets. Over a small number of spins these bets are more likely to conserve the bankroll. These bets are for players who are more interested in experiencing the thrill and glamour of playing roulette rather than in winning big money. The even money bets are low risk low return bets.
On the other extreme is the single number bet in which the player bets on a single number. As against the even money bets that win if any of eighteen numbers are called this bet wins only if the specific number is called. Therefore this bet wins less frequently. The single number bets payout at 35 to 1. $1 wagered on these bets wins $35. These bets are for players who do not mind losing the bankroll, but if they win they would like the payout to be substantial. Players who go for these bets are more serious gamblers who can start with larger bankrolls and are willing to spend more time at the roulette table. The single number bets are high risk high return bets.
These are the two extremes and roulette players have a large number of options in between these. Hence they can choose the risk return level they are comfortable with. The standard bets in roulette cover one (Single), two (Split), three (Street), four (Square), six (Six), twelve (Column) and eighteen (Even Money) numbers. That gives the players six levels of risk return from which to choose. But the player is not restricted to these six levels. Suppose a player finds that covering nine numbers is an appropriate risk return level for him. He has a variety of options such as covering nine separate single number bets or a six numbers and a three numbers bet or two four number bets and a single number bet. This is what makes roulette an interesting game.