Aug 2, 2017

How Slot Machines Work

author Peter S.

Online casinos offer an unbelievable number of different slot machines, so you should first take a look around and find the ones you like. In general, your goal is always to achieve a winning combination - although different games have different rules. Here you can take a closer look at the game process.

Table of content

History of Slots

Let us start from the beginning. The inventor of the first slot is a man named Charles Fey. Sometimes also called the "father of the slots". Charles came from a large and poor German family. He was born in 1862. Around the age of fourteen, he began to show a strong interest in mechanics. He gained valuable knowledge in this area in the first electro-mechanical factories in France and the United Kingdom, where he worked for several years. However, he was not very satisfied with the life of the employee in the factory, so he decided to move to the USA. He was only 23 years old when he got a job at American Electric Company. Here he met his future business partner Theodor Holtz. Together, they decided to leave work in the factory and set up their own business for the production of electro-mechanical equipment.

For the avoidance of doubt, money could not be won on the very first devices that could be called slot machines. You could play games with poker cards (understand: you could play hi-card, later poker draw, not real poker) and were developed by the New York Company. The devices were called Sittman and Pitt Company. Instead of money, you could win food, drinks, chewing gums or cigarettes.

The First Slot Liberty Bell

Liberty Bell slot

The first Liberty Bell slot had a total of three steel reels (originally called hoops). The machine contained ten painted symbols on each of them. Namely, there ware Liberty bells, horseshoes, spades, diamonds and hearts. However, they did not have much in common with current game symbols. To put the machine into motion, the player had to toss a few coins and pull the lever. It started the game round (the reels started spinning) and you just waited for the reels to stop. Winning combinations were always made up of three identical symbols in a row (two were exceptionally enough for a small payout). If you won, the bell (hence the name) allegedly rang. The win was around fifty cents.

Logically, it took several dozen years for slots to transform into what they look like today. Fey's successors developed new slots by gradually adding the number of reels and symbols on them. Of course, the design also constantly evolved. Demand was indeed unusually high at the time the slots were created. Fey leased his slots to local stores, receiving a fifty-per cent commission fee on profits for the rest of his life.

The paylines looked like this:

  • 2 horseshoes = .05 cents
  • 2 horseshoes + 1 star = .10 cents
  • 3 carts = .20 cents
  • 3 crosses = .30 cents
  • 3 hearts = .40 cents
  • 3 Liberty Bells = .50 cents

Gradually, however, serious problems with the law arose as slots began to fall into the category of gambling, which was banned. This forced Fey to re-install non-cash prizes winnings such as food and drink in some of his devices. However, fate did not seem to be in favour of the slot machines. At least as far as the laws were concerned. Along with the great wave of popularity, the legislative wave also rose, and slot machines were outlawed in 1909 in San Francisco and later in Nevada in 1910. A big surprise was the ban on slots in 1911 in otherwise liberated California.

Evolution of Slots

Neither Fey nor his successors ever stopped improving the invention. Most of the changes were in the design - Fey was the first to install lighting in the slots and generally improved the design in such a way that the slots fit better into the environment (bars, cafes and casinos). Fey himself realized that it was important to place the slot close to the entrance to immediately alert incoming visitors. Other changes were mainly about the material. The original iron was replaced with wood. Some improvements also came involuntarily. The first cheaters appear and started using counterfeit coins. At the time, the slot companies came up with another of the inventions - the coin separator, which could recognize counterfeit coins.

Fey's factory did not manage to fulfil the demand for slots, but Fey refused to sell his patent for production to anyone else. Despite this, in 1907 a mechanic named Herbert Mills from Chicago decided to produce the same machines under his own brand - Operation Bell. But every cloud has a silver lining. And so with the arrival of Operation Bell on the scene in 1910, the slots underwent a number of significant improvements by the Mills Novelty Company. As part of this upgrade, they experienced a smarter entry for coins and, above all, the introduction of the popular "fruit" symbols, such as cherries, plums or lemons.

The greatest contribution to the popularity of slot machines was the advent of video slots in the mid-1970s. Due to the fact that video slots are computer program based and do not have mechanical reels, developers are able to implement more paylines, symbols and betting opportunities. These modern online slots thus have a number of features that are not possible on mechanical reels. Thus, you can nowadays encounter various wild and scatter symbols, free spins, bonus rounds and other gadgets.

💡 The first online slots were released in 1996 in InterCasino. It was the first online casino with 18 online slots. The first online slot developer was Microgaming, which still makes games today.

A by-product of this situation is the fact that casinos can offer bigger jackpots thanks to more lines and betting options. Henceforth, players all over the world can win jackpots that are even 6 to 7 digits in size. The trend of increasing jackpots has continued and some casinos have had casino jackpots in the tens of millions. This all culminated in 2013 when an unnamed Norwegian won an incredible jackpot worth €17,861,813.

Online casinos began to appear in the mid-1990s, giving players the opportunity to enjoy their favourite gambling games from the comfort of their own home. On the other hand, accessibility increased to the point where almost anyone could get in on the fun. Today, you can play anytime and anywhere, perhaps on your mobile or tablet.

Slot machines - rules of the game

If you've never played slots before, you'll be wondering when you'll win on slots. A win is credited as soon as identical symbols appear on the reels from the left in a particular line. The number and type of these lines are not always the same. Some older slots have only one winning line. Some slots, on the other hand, have up to thousands of lines. The amount of the win is then calculated according to the symbols that make up the line. Symbols that are fewer in number on the reels pay higher wins and vice versa. Virtually all games have a different number of lines and different payouts.

Example pay lines

how slots work - paylines

Example payouts

how slots work - payouts

The game itself is very simple. All you have to do is select the bet amount and press the Play or Spin button. The slot machine will spin the reels, which will stop again after a while. If a payline is formed, you win. Online slots also offer an autoplay button. In case anyone gets tired of pressing the button after every spin. You can choose how many spins the game will play or also, for example, that the autoplay will stop when you win or when the bankroll drops or rises to a certain level, or when a certain game feature is activated, example.

It can be slightly confusing for new players to select the bet amount. This is because you can set the coin value, the number of coins bets per line, and the number of lines. At least for some slots.

how to select a bet amount

The most important thing is how much the resulting amount will be. This is calculated very simply:

Coin value*Number of coins*Number of lines

If the coin value is 1, the number of coins wagered is 1 and there are 10 pay lines, the resulting bet is €10. For example, if the coin value is 0.5, the number of coins bet is 5 and the number of winning lines is 10, the resulting bet is €25. If you have a problem with this, simply set the coin value to 1.

This system is most commonly found in slots that have only a few pay lines. For games that have dozens or hundreds of winning lines, you can only change the total bet amount. Which makes everything very simple.

More or fewer lines - which slots are best?

At first glance, it may seem that playing slots with more winning lines can be more profitable. More chances to win equals more wins. While this is true, games with fewer pay lines generally have higher payouts. The truth here is that how much you win or lose on a slot machine is not determined by the number of pay lines. The most important thing is the so-called RTP - or Return to Player.

⭐ For more on how RTP is calculated and the chances of winning, see the article about the Probability of slots.

If you want to find the slot machine with which you will lose the least on over the long term, find the game with the highest RTP. This is because this number tells us what percentage of the total bets will pay back to the players. The average online slot machine has a return rate of around 95%. Which is comparable to, for example, American roulette.

In the end, it's up to your preferences. Do you want to win more often but in smaller amounts or less often and larger amounts? If you play for a long time, it works out financially the same. 

What features and bonuses online slots offer

Special gaming features and bonuses are an integral part of modern online slot machines. They make the game more entertaining and may add a few extra credits to your bankroll. Although many slot machines are completely unique, there is a set of elements that you will encounter in most games. 

Wild Symbols

how slots work - wild symbol

So-called Wild Symbols are the most widely used game element in slot machines. These symbols can replace any other symbol to create extra paylines. Wild Symbols is exactly where the most innovations can be found. These substitutes can travel with each turn on the reels or they can stay locked in one place for a few turns. You may also encounter Wild Symbols extending to the full height of the reel or get a multiplier for playlines containing these symbols.

Scatter Symbols

how slots work - scatter symbol

You can also often find so-called Scatter Symbols. These are symbols that can usually be found even on 5 reels and they pay out prizes even if they are not grouped on a payline. They often trigger other game features such as free spins or pick-and-win bonuses. Some old fruit slots also offer Scatter symbols but in these games they just pay a win without triggering any bonus feature.

Free Spins

how slots work - free spins

As you can guess, free spins are games that do not cost you credits. But their winnings are real. These free spins are most often triggered by special symbols or above-mentioned Scatter Symbols. There are several types of Free spis. You can have free games in the main game, or the special bonus round is triggered. These bonus round with free spins can also include more bonus features.


how slots work - Pick and Win

When Pick and Win bonus is triggered, the player gets to choose from several options. There may be visually represented as treasure chests, doors or anything else. Each of these options hides winnings or other benefits. Just like with free spins, Pick-and-Win is also triggered by 3 or more Scatter Symbols.

Cascading Reels

how slots work - cascading reels/avalanche feature

Cascading Reels are one of the latest gaming features in online slot machines and there are only a few games offering them at the moment. Once a payline is created, the active symbols are removed from the game field and are replaced by other random characters. That gives the player another chance to win. Sometimes are cascading reels called Avalanche feature.


how slots work - gamble feature

On the other hand, the Gamble feature is one of the oldest available and is well-known by players of classic mechanical slots. If the slot machine offers Gamble, players may choose to double or even quadruple their winnings by guessing a card or colour.

Colossal symbols

How slots work - colossal symbol

Collosal symbol is a game feature that makes symbols appear on the reels that cover more than one classic field. When they appear on the reels, they count as two symbols below and two symbols next to each other.

Cluster wins

How to win - cluster win

In classic slots, you win when identical symbols appear on the selected payline. However, some online slots do it differently. They have more reels and more lines. There are no set paylines, but players win as soon as at least, say, five symbols are adjacent to each other. So they form a kind of cluster.

Two way wins

How slots work - both way win

Online slots with the Two win way feature work quite simply. Wins are counted from both sides. So it doesn't matter if the payline appears from the left or the right. Most slots only count paylines that start on the leftmost reel.

What Types of Slots Exists?

If we look into the world of big gambling, whole rows of these "one-armed bandits" (a popular term in the US) are a crucial part of every casino in Las Vegas. If you want to play and try out a variety of free slots, our site offers a wide range of options. If you like or are just about to play, you should keep in mind that there is no slot like a slot. There are several types of slots, and it is very important to choose the ideal one. The most common types include:

🎰 Classic Slots - these are the original 3 reel machines dressed up in a digital coat. These games were created in the 21st century and incorporate the latest technology. They have multiple pay lines and a perfect old school look including the typical clicking sound.

🎰 Progressive slots - they are one of the legendary slots and are popular especially abroad. The volume of all bets adds up to create a huge jackpot. Once someone hits a special combination of symbols, they can start enjoying their millionaire dream.

🎰 3D slots - are similar to video slots, but even more focused on your feelings. Everything takes place in 3D reality. 3D slots have great design and visual features. One of the most famous 3D slot providers is Betsoft which created the most famous 3D slots in the world. 

🎰 Multiple-payline slots - Older and fruit machines usually offer only a few paylines. Newer games, however, have thousands of paylines. All variations are available. While you win more often with these slots, the payout per line is usually lower.

🎰 Virtual reality slots - With the advent of new technologies, players can also play slots in virtual reality. Some software providers are already releasing slots that can be played in this virtual reality.

🎰 Mobile slots - Due to touch screen technology and smaller screen sizes, online slot manufacturers have to modify the game interface. For example, they have to lay out the buttons differently or modify the graphics to make the online slot machine playable on mobile.

How Slots Work - The Autopsy of the Slot Machine

Let's start from the already proven fact that not only online slots but also slots in casinos and gambling halls are controlled by computer software. And this software contains something called a random number generator algorithm. The regularity of the operation and randomness of slots is checked by independent audits, based on which licences are granted to game manufacturers.

"A 'fair' payout system is provided by the slot machine's GCB (Gaming Control Board). This crucial technical element, which is contained in every slot (and is naturally also part of the online slot machine), is of course subject to strict audit by state control.

It's interesting to see how such an audit of a slot machine takes place. Firstly, the proper functioning of the coin machine (for physical machines) is monitored. This is checked approximately once a week. Once a month, casino managers' reports are checked. Any deviations detected are to be dealt with as quickly as possible - i.e. within the time available to the officials. Just for the record. Moreover, these checks are run by the casinos themselves! The moment they do not match the results from the official GCB test, the casino may lose its license.

The brain of the slot machine is the random number generator. After the familiar process of the machine verifying the authenticity and value of the coin and pressing the button to start the game, the generator draws a number to which a certain symbol on the reel is associated. On this basis, the slot machine's braking system takes care of stopping it.


For example, such a 32-bit generator selects numbers between 0 and 4 294 967 396. This number is then compared to a memory device in the computer called EPROM (Erasable Programmable Read Only Memory), which then outputs the result for reels 1, 2 and 3. Only after this logical operation is completed will the individual reels start spinning! The well-known "pushing the reels with the eyes" has therefore no real justification in the field of slot machines.

The S1 reel spins, usually for one second, to perform a forced spin of at least one round - until the virtual stop assigned to a particular symbol stops directly on that symbol, all thanks to a "stepper motor" that drives the reel and can "safely" stop it at the desired position. In the meantime, of course, the second reel is also spinning, and after a predetermined number of courses (which also takes approximately one second), it too stops on the desired symbol. The same applies to reel number three. Precise stopping is achieved using an electrical signal that each physical symbol emits, and if the signal matches the desired one, the computer evaluates this and then mechanically stops the reels.

This is because the generators are oriented as unpredictable "towards the left" - i.e. so that it is not possible to estimate the next number based on previous results. However, "rightward" - predicting previous generation results based on later results is theoretically possible. In these systems, the problem mentioned above is treated by continuously generating random numbers. This is whether the machine is in operation (the reels are spinning and you are betting) or not. Thus, nobody knows at which time which number was generated. However, this sequence of numbers has a clearly defined time at which the process takes place and starts repeating. If the slot is running the random number generator thousands of times per second (roughly the case in normal practice), then one sequence lasts for 50 days and then the number generation starts from the beginning.

RNG - Random Number Generator

There are three basic types of random number generators:

  1. Toys. These very simple random number generators, which are programmable in almost any language, are usually used only for small computer simulations such as "loading line" etc.
  2. Slightly better 32bit and 64bit generators, which are used in slots and video games
  3. Real random number generators that use electronic circuits to verify laws of nature. These generators are mostly used in research (from calculations with the human psyche, to the military, to speculation in financial markets).

Try random number generator for free. Choose the range of numbers, the number of numbers you want to generate and click on the Generate button. The random numbers will be generated below the input fields.

Random Number Generator

The whole principle of how slots work can be summarized as follows:

The player presses a control button, the random number generator is instructed to pick a random number. At the same time, the reels spin. It takes a while for them to "stop" again, usually around 3-5 seconds. For those 3-5 seconds you are watching a completely fictional process. All the numbers generated by the generator are already evenly distributed on the individual symbols on the reels before the game starts. The reels will "accidentally" stop on the exact symbol to which the number belongs. The selection of the number is indeed completely random. It cannot be influenced in any way. This is important, so we will repeat it. The choice of number is not affected by anything. Each "spin" of the machine is completely random and there is no difference between them.

The proper functioning of the RNG (Random Number Generator) and the randomness of slots brings up some interesting implications that most players, unfortunately, don't understand.

Thus, there is no such thing as a "pay cycle" in which a slot machine pays out larger wins than normal. There can't even be a situation where the slot machine seems to be "just about to pay out". It is important to remember that all casino games - from craps to slots to roulette - work the same. The outcome is always a matter of chance, and the odds, or probability advantage, is always on the casino's side. There are no arcane practices that can in any way guarantee you a win. It is the casino's wish that the slots work just like any other game - completely random with a certain percentage advantage to the casino. So it is very hard to win at slot machines. It is simple matter of luck.

You will also often come across the myth that if you play slots for free, the winnings are much more frequent than if you play for money. However, this is just an illusion, which is caused more by the human psyche or a work of chance. All of the most famous and popular casino software providers have all the certifications, and of course, this option is thoroughly tested as well.

An Example

To illustrate this, it is a good idea to imagine a reel with individual stops. You can see this on the right. It has a total of 22 stops. Yes, even the empty boxes are stops. At first glance, it may look like if you have 11 symbols and 11 empty spaces, you have a 50% chance of hitting one of the symbols and a 1 in 22 chance of hitting the jackpot. Wrong! In practice, we have a virtual reel with a much larger number of, for example, 128 symbols. So the computer picks a number from 1 to 128, each of which is assigned to a specific stop. For better illustration, let's look at the following figure.

Number Symbol Total number of symbols
1-73 Empty 73
74-78 Cherry 5
79-94 Bar 16
95-107 Double Bar 13
108-118 Triple Bar 11
119-126 Red 7 8
127-128 Jackpot 2

Let's say a random number generator picks the number 53. In this case, it will hit on an empty box. If he picks 127, the jackpot will hit. As you can see, some symbols are simply more likely to "appear" than others. Although, at first sight, it may seem that their number on a reel with winning symbols is completely balanced. Unfortunately, what is very important to note is the fact that a total of 73 out of 128 symbols are non-winning. You therefore have a 57% chance of hitting one of them.

Speaking of non-winning (empty) symbols, you'll also find that some symbols tend to land a bit more often than others. Like they're "altered"? And you'd be surprised! It is! If you look carefully at the following table, you will see how different symbols can be altered.

Stop Symbol Number Odds
1 cherry 1-2 2
2   3-7 5
3 8-12 5
4   13-17 5
5 7 18-25 8
6   26-30 5
7   31-35 5
8   36-41 6
9   42-43 2
10   44-49 6
11 == 50-56 7
12   57-62 6
13 cherry 63 1
14   64-69 6
15 = 70-75 6
16   76-81 6
17 82-87 6
18   88-93 6
19   94-104 11
20   105-115 11
21 jackpot 116-117 2
22   118-128 11

The fourth column in the previous table shows just that "alteration". Some symbols are favored. Note how the empty symbols near the jackpot (and other valuable symbols) are jingled. The distribution of the number of virtual stops around valuable symbols is very numerous. This makes the probability of hitting the numerical range of these non-winning stops very high. In addition, this produces a pleasing "close-run effect" at any given moment, which gives the player a psychological boost in the game. It can feel like there are magnets attached to the symbols around the jackpot. But in reality, you weren't even close to the jackpot, it was a real illusion.

No Two Reels are the Same

So far, we've talked about how the symbols and virtual stops are positioned on each reel. However, slots have at least three reels, and what's interesting, and what might put you off playing slots for good, is the fact that each reel is altered differently! Moreover, reel to reel, the alteration is more pronounced around the winning symbols. This causes an even more frequent and more pronounced "tight-lead effect". How many times have you had a jackpot-jackpot on the first two reels and an empty box on the third? Just think of the excitement when the jackpot symbols land on the first two reels and stop just above/below on the third reel. Unnecessary holding of breath and rolling the reel with your eyes. Just remember that you have much lower odds on the third jackpot than on the first two. 

You also remember an important fact at the end. The random number generator is constantly picking a numbers, even when no one is playing the slot. Why? Simply because it becomes completely unpredictable at that particular moment.

How Progressive Jackpots Work

In online slots, where the payouts are random at different levels, the values are rarely ever published. There are exceptions here too, but in most cases the odds are unknown. In this respect, we can only guess. Why? Because we simply are not able to play enough games on a single machine to know its frequency. So if we hit the jackpot, it's just pure chance. A game that looks very promising to one player is uninteresting to another.

The accompanying table, which tells us that our slot machine has an average return of 92.61%, only serves to show us that in the short term we will never reach that figure and hides the true characteristics of slots.

If we take into account all possible combinations of stops on each reel, a single game has 373,248 possibilities to create a final combination. Therefore, a player would have to play more than 373,248 games to get relevant final combination results.

🎰 The highest slot machine jackpot was hit in 2003 in Las Vegas. An anonymous software engineer won $39 million that year.

🎰 The highest online slots jackpot ever hit in 2015, when UK player won $20,062,600.

What are the Actual Odds to get Jackpot

For example, in 373,248 games, which we can call a cycle, the jackpot will be hit, paying out at an average of 833 to 1 on 8 occasions. Considering that a round lasts five seconds, we play 130 games per hour. Our chances of hitting the jackpot at this point are 13.96 percent. While the odds of not hitting any jackpots are much lower, at exactly 0.03 percent. So our chances of hitting eight jackpots are 45.30 percent and 40.74 percent, respectively.

Probability and benefit relative to the return percentage of online slot machines. 

Suppose you are playing a 4-hour session. You pace yourself and play one game every 5 seconds. In total, you play 2880 games. All in all, the chances of not hitting the jackpot are a full 94.01 percent. That means you'll hit it in a pretty good 5.80 percent of games. Don't despair, it's not much, but it's better than nothing!

Let's let the top jackpot of 833 to 1 go and take a look at our chances of hitting the second highest. Its payout is 320 to one and we won't hit it in 78.12 percent of the games. But in that case, we have a great 19.29 percent chance of hitting the jackpot. Two of each will then hit us 2.38 percent of the time.

Small Versus Big Jackpot

Note that the highest jackpot (833 to 1, that is, 2,500 credits for a bet of 3) is generally considered low. Many other online slots offer far more favourable payouts. Here, however, you need to be on your guard. The higher the possible payout, the lower your chances of achieving it.

Let's say our favourite slot machine pays out a jackpot of 3333 to 1 (10000 for 3 credits). Our conditions are the same as in the previous example. Now we hit 2 jackpots in 319928 games and lose in 319928 games. So the percentage of return would be the same except for two decimal places.

At a payout of 3333 to 1, we will fail to hit the jackpot 98.47% of the time out of 2880 games. Our chance of a successful round is therefore only 1.52 percent. Our chances of hitting 2 or more jackpots are only 0.01 percent.

Another indicator is the percentage of effective return in case you don't hit the jackpot. In both cases, the chance of hitting the jackpots contributes 1.78 percent to the return. So instead of 92.61 percent, you will only miss the jackpot 90.83 percent of the time. If you've never hit but the second-highest jackpot, your chances are even worse.

Other games of chance have far fewer possible outcomes. Since there are fewer possibilities, you need to approach the game even more sophisticatedly. Never forget this, the laxer you play, the more advantage the casino will have.


The following question is obvious. So what are the odds of hitting the jackpot? As we mentioned earlier, the odds of hitting the jackpot are 2 in 128 on a single reel. The probability of hitting three jackpots on three reels is given by the following logical calculation: 2/128 x 2/128 x 2/128, numerically 1 in 262,144. To give you a better idea, here is a practical example. If you play eight hours a day and manage a very respectable 800 spins per hour, you should hit the jackpot approximately every 41 days.


We already know the odds of hitting the jackpot. Now we want to know what the payout probability is. That is, how much the slot machine returns on an infinite number of spins. Of course, in practice, you can't afford to play that long. However, the longer you play, the closer you get to the slot's set long-term payout.

Let's assume that the pay table of our machine is as follows. To get the final payout calculation of the machine, we need to multiply the probabilities of each winning spin and add them all up, finally dividing by the total number of spins.

The total payout for this machine is therefore 96.30%. For every penny you throw into the machine, you will get a total of 96.3 pennies back in the long run. It is quite interesting to see which symbols contribute to the total payout back to the player. The most generous are the "cherry" or "any bar". The jackpot contributes less than 1% of the total payout.

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