Traditionally, slots have used mechanical reels with one to three symbols per reel. While many modern machines use a random number generator instead of physical reels, most still employ a lever or button (either real or virtual on a touchscreen) to activate the machine and start spinning the symbols. If the symbols form a winning combination, the player earns credits based on the paytable. Most slot games have a theme, with classic symbols including fruit, bells, and stylized lucky sevens.
In the 1980s, manufacturers began to incorporate microprocessors into their machines, allowing them to program each symbol with a different probability of appearing on a given stop. The odds of a losing symbol seemed to appear more frequently than it actually did, leading some players to think they were close to hitting a jackpot, when in fact the probability was much lower. Similarly, the appearance of multiple winning symbols on a single reel may have led players to believe they were getting close, when in reality the winning symbol had already appeared several times.