Slot machines are gambling machines in which a player places a bet, presses a button and sees a random result on spinning reels, which decides whether the player wins. A modern slot machine is a computer in a cabinet. Its reels are usually pictures on a screen, and each game on it is software. In Britain, slot machines are also called fruit machines, and in Australia they are called pokies.
The result of each spin is decided by a random number generator, or RNG. This is a program that produces numbers by chance. The RNG runs all the time, producing thousands of numbers every second, even when no one is playing. When the player presses the button, the machine takes the number produced at that instant. The game's software converts that number into a position for each reel. So the spinning reels only show a result that has already been decided.
Take, for example, a slot machine in a casino in Las Vegas, in the American state of Nevada. Its game is set to pay back 92% of the money played. This share is called the return to player, or RTP. Say players bet a total of $100,000 on the machine in a month. Over that month, the machine is expected to pay back about $92,000 as wins, and the casino keeps about $8,000. That 8% is the house edge. Any one player may win or lose much more than 8%. The share holds only over a very large number of spins. Nevada's regulator requires every machine to pay back at least 75%.
Before a game may be used, an independent test laboratory checks its RNG and confirms its RTP. The regulator then approves it. After that, the software cannot be changed without a new approval.
The machine does not work alone. It reports every bet and every win to the casino management system. It can accept money from a player's phone through cashless gaming, as well as notes and paper tickets. The maker either sells the machine to the casino, or places it on the casino floor and takes a share of what it earns.
The same game is often released as an online casino game too. It may also appear as a social casino game, played with coins that are never paid out.
In India, slot machines are legal only in the casinos of Goa and Sikkim. But engineers in Pune, Bengaluru and Chennai build many of the games that run on slot machines around the world.