Slot machines were, historically, gambling machines with 3 or more reels that spin independently when a button is pushed. When the reels stopped, the machine would pay out according to how the pattern of symbols displayed on the reels matched with the symbols on the pay out tables displayed on the machine. The modern version of slot machines are computerized so that the position that the reels stop on are now chosen by a Random Number Generator (RNG) instead of by the physical motion of the reels themselves.

The theoretical payout percentage of slot machines is 82-98%. This means that 82-98% of all the money wagered by players is returned to the players as winnings. The minimum payout percentage is usually determined by law in the specific location of the casino