How Slot Machines Work

Slot machines are the most popular game in casinos. Though most of us have played them before, few know the processes that go on within the machine to make that spin happen. In this article, we are going to cover how slot machines work, how they have evolved through time, and the different types of slot machines you can find today.

History of Slot Machines

Antique Slot MachineThe slot machines we know today came from a long evolution of the game. In fact, the first slot machine dates all the way back to 1895. If you want to look back to the very beginning of automated gambling devices, you could actually say the first machine of this type dates back to 1891.

Charles Fey was a mechanic in San Francisco, California. He created the first slot machine, called the Liberty Bell. The slot machine contained 3 spinning reels, each containing 5 symbols; a horseshoe, heart, spade, diamond, and Liberty Bell. The highest payout awarded was 10 nickels, for getting 3 Liberty Bell symbols. This was the first gambling machine to offer automatic payouts.

Though not considered to be the first slot machine by experts, a similar gambling machine was invented four years earlier in 1891. It was developed by a New York company called Sittman and Pitt. Their invention was a poker machine, that was played like a slot machine. There were 5 reels, with a total of 50 cards inside. Two cards were removed to add a house advantage; the ten of spades and the jack of hearts. A payout was made when the players spin resulted in a winning poker hand.

There were no monetary payouts and the prize won was decided by the establishment the game was located in. This was usually at a bar and the prize was drinks or cigars.

Two other significant advancements in slot machine technology were; the first electromechanical slot machine in 1964 and the first video slot machine in 1976.

Modern Slot Machine Mechanics

Today’s slot machines are much more complex. They have evolved from mechanical games to computer controlled software programs. In fact, the reels displayed on modern video slot machines are more of a formality than a part of determining your win.

The symbols displayed on the reels of video slot machines are determined from the microsecond you pushed the spin button. At that point, you stopped the Random Number Generator. Each symbol on the reels is decided before they even start spinning.

The house advantage, or your odds of winning, are predetermined and programmed into the software of the slot machine. That does not mean that you will lose, or potentially win a set amount. It simply means that in the long run, the house will take a predetermined percentage of your money.

Random Number Generator

All slot machine results are based on a random number generator, also called an RNG. This is a microprocessing chip installed in the slot machines hardware. This computer chip is the sole piece of equipment determining if you walk away with a jackpot win on that slot machine.

So how exactly does this work? A random number generator is constantly producing a series of random numbers. This is done at a rate of several billion per second. The random number from the RNG correspond to a specific position of the slot machine reels. When you hit the spin button the Random Number Generator stops. Whatever number the RNG is displaying at that time, and the corresponding reel symbols for that number is displayed on your slot machine screen.

The Random Number Generator uses a mathematical algorithm. It begins with a number, called a seed. From there it completes a series of multiplication and division equations, finishing with a new number. This is then repeated. Then its repeated again, several billion times per second.

The fairness and randomness of Random Number Generator software is extensively and continuously tested. There are independent testing agencies, as well as testing agencies associated with the gaming control board of the area in which the casino is located. Random Number Generators assure the player that they are getting a fair game.


Slot machine variances can be listed in several ways. Each of the following categories can build upon each other. For example, there are several different denominations available within 5-reel slots, that are also progressive. Below is a list of several common slot variations:


Slot machines can be played in several different monetary denominations. This means that 1 credit is equal to a different monetary value. On a penny machine, one credit is equal to $.01. There are also $.05, $.25, $1.00, and even higher denominations. Statistically, the higher the denomination, the higher your odds of winning.


Slot machines can have a varying number of reels. More traditionally, slots had 3 reels. Today, many video slots have 5 reels. There are also 6 and 7-reel machines.

Flat top vs progressive

Progressive slot machines are a network of slots all tied together vying for a very large jackpot. This means that you could be playing a progressive machine in one state, while someone in a different state is competing with you for the same jackpot. Because casinos can link so many machines and venues together, they are able to offer incredibly large jackpot amounts. Some of these rival national lottery jackpots.

A flat-top slot machine is a slot with a capped jackpot amount, exclusive to that game in that casino. These are much smaller than a progressive jackpot. The maximum amount you can win is specific to that game.


Paylines refer to a set line the symbols can follow on the slot machine reels. On the older, traditional slot machines, there was just one; a single payline running across the center of the reels. Today, there are typically 20 or 22. There is even a casino slot that has 243 paylines. This is because there are actually no paylines. The slot symbols can line up in any order to initiate a winning spin.

Game Type

There are two main slot types today, at least as far as gameplay is concerned. These two types are classic and video slots.

Classic slots are usually 3-reel machines. They will usually have 9 paylines, or sometimes even 1. This is to mimic the traditional slot machine style. The classic slot machine will also have actual reels that spin within them. These are still controlled by a Random Number Generator.

Video slots make up more than 80 percent of modern casino floor slot machines. They are increasingly popular due to their exciting game features. These are usually 5-reel machines. The majority of new slot machine games are video slots.

Fun Facts

Slots in JapanJapan takes the crown for the highest number of slot machines. They have approximately 5 million machines.

Compare that to the US at 800,000 slots.

In the early days of slot machines, a popular payout was sticks of gum. The logo of Bell Fruit Gum was a black rectangular bar. It is that logo that inspired the “bar” symbol we still see today in modern slot machine games.

IGT’s Megabucks game was the first to link machines at different casinos to offer multimillion-dollar prizes. This was the first progressive slot machine game.

Slot machines are referred to as different names depending on where you are geographically. In the US they are almost exclusively referred to as a slot machine. In Australia and New Zealand, they are called “pokies”. The UK calls them “fruit machines”, due to the common representation of fruit symbols on the reels.

A classic name for a slot machine is a “one-armed bandit”. This term was originally used to describe people who figured out how to steal money from the games. It later became a term to refer to the slot machines themselves. This was due to the machine taking money from players. The one arm bandit term was referring to the lever used to spin the reels.


Slot machines have come a long way over the years. The introduction of video slots opened a whole new aspect of entertainment within slot machines. Bonuses, progressive jackpots, video clips, etc. all add to the entertainment value. This is all because of the advancement in technology, making slot machines more of a computer software program than a machine.

One aspect that players seem to take for granted is the security involved in slot machine play today. With the advancement in slot machine programs, has come the advancement in slot machine regulation. The introduction of the Random Number Generator software has implemented a standard of fair gameplay. With software tracking systems, you can be assured that the slot machine you are playing is being controlled with honest practices.