During the late nineteenth and early twentieth centuries, slot machines were popular in resort areas. During the Great Depression, however, slot machines were banned in San Francisco and many other cities, and distribution was often controlled by organized crime. Legislation to control the distribution of slot machines was passed, but this was often ignored by casino owners. Slot machines have since become one of the most popular games in casinos, generating more than 60 percent of the gaming revenue in the United States.

The earliest types of slot machines had a horizontal line across the front of the machine, which was the payline. When a specific symbol fell on this payline, the player would win a prize. While the payout amount has decreased over the years, the basic concept of slot machines remains the same. The player pulls a handle to rotate reels with pictures on them. Depending on the pay line, the player can win a prize if all of their winning combinations appear on the payline.

Modern slot machines are controlled by computers, instead of gears. Though they may look similar to the mechanical machines, the principle is different. Instead of relying on the movement of the reels, the machines are controlled by a computer program. This allows for greater flexibility and customization. When players want to play more of the same game, they can customize the payout frequency by changing the settings in the computer. There are numerous settings available to make the machine pay out more often or less often.