Originally a slot machine was a simple device that spun a set of reels and accepted coins. However, over the years, slot machines have evolved into sophisticated electronics. Modern slot machines use microprocessors to process symbols and assign probabilities. Some games include bonus rounds, interactive elements, and advanced video graphics.

The most common types of slot machines are three reel machines. These machines typically have one, three, or five paylines. A three reel machine is simpler and more reliable, although the payouts may be lower.

A multi-line slot machine is more advanced than a traditional three reel machine. Several symbols can be represented on each reel, resulting in thousands of possible combinations. Multi-line machines may also accept variable credits, such as 1 to 15 credits.

Depending on the game, some symbols are only visible on certain reels. Wild symbols may also appear on certain reels, substituting for most other symbols. A wild symbol may also have a chance to “stack” across the entire reel.

The earliest mechanical slot machines used five reels. These machines also included modified reel-stop arms that allowed early release from the timing bar.

In the mid-1920s, slot machines were available only in small shops and casinos. However, in the 1990s, slot machines began to become more popular. Some of these machines are now available for sale on the Internet.

Modern slot machines no longer use tilt switches. The term tilt is still used when a technical fault is found.