Game Development – How Are Online Slot Machines Constructed


How are online slot machines constructed – All parts you need to implement when developing a new slot game. A brief overview.

An online slot game usually carries many different symbols. Those symbols, like fruit or other items, are displayed on rows and reels. A row is the horizontal arrangement of symbols. A reel is the vertical arrangement of the latter. A common slot machine can consist of three rows combined with five reels. Below the symbols there are usually the buttons with which the player handles the game: A button for setting your bets before each round, another one for triggering the spin rounds. Another button lets the player set automatic spins for some rounds, if your don’t want to activate each round manually. Excellent examples of such online slots can be checked out in this Stake casino review.

slot machine development

The different Slot Symbols

After we have dealt with the topic of reels and rows before, we now want to go into the individual slot symbols. Of course, it depends on the individual slot machine. In general, however, symbols can be divided into three different categories:

Low Value Symbols

These symbols are part of the slot machine, but with the right arrangement provide for rather smaller wins. Usually, these symbols are displayed as playing card symbols. For example, such a symbol can be a “10”, a jack or a queen.

High Value Symbols

These symbols are more valuable than the playing card symbols. As a rule, these symbols are adapted to the theme of the slot machine. For example, in an Egyptian slot machine, a higher value symbol could be a pharaoh. If you spin these high-value symbols in a winning arrangement, you can expect correspondingly higher winnings.

Special Symbols

In addition to the two types of symbols mentioned above, slot machines usually have special symbols. Here, we can again distinguish between three groups of symbols:

Wilds Wilds are usually extremely valuable symbols. As a rule, wilds replace all other symbols and help you win. Before you start a new slot, you should take a look at what the wild symbol looks like in the game and what function it has.

Scatter symbols are also very valuable along with Wilds. Usually, scatters help to activate a special slot feature. For example, three scatters spun at the same time can activate the slot’s free spins mode.

Bonus Symbols Some slots offer other bonus symbols besides wilds and scatters. These symbols also usually activate a bonus game or an additional feature. Before you start a new slot machine, just look at the game instructions of the machine. There it will be stated whether there is a bonus symbol or not.

For a beginner, the different types of symbols may sound a bit complicated. However, with the right casino tips, the function of the various symbols can be understood very easily and quickly.

How to Win on Slot Machines?

Now that we’ve looked at the structure and functionality, as well as the different types of symbols, let’s take a look at how to win at slot machines. Here, too, there are various possibilities that vary from game to game. We will now briefly introduce the most common variants:


This is probably the most popular way to win at slot machines. A slot machine has a different number of so-called paylines. A payline is a horizontal line that is laid across the slot. When you spin a certain number of the same symbols on one of these lines, a win is triggered. The number of these paylines can vary from slot to slot. Usually, the number is between five and 100 paylines.

The number of paylines is indicated in the game instructions of each slot machine. In addition, it is further specified exactly how these paylines run. The higher the value of the symbols you spin on a payline, the higher your round win will be.

Adjacent Symbols

A second popular option for determining wins is the option of adjacent symbols. Here you win if you get a certain number of symbols on adjacent reels. As a rule, these adjacent symbols must start with reel one. Three matching symbols on three adjacent reels will give you a round win. Depending on the structure of the slot machine, this results in a certain number of winning opportunities. As a rule, this is around 256 ways to win.

Cluster Wins

A third variant to achieve winnings is not so popular, but is used by more and more software manufacturers. This is the possibility to achieve winnings in clusters. For example, you win when you get five of the same symbols in a cluster. A cluster is a contiguous area on the playing field. It can be vertical, horizontal and diagonal. When a slot machine pays cluster wins, we recommend that you read the game’s instructions and the corresponding cluster rules.

If you are new to online slots, you should briefly read the game instructions before each game. There, the exact rules for round wins are always explained very well and very precisely.

Special Features – The Winning Helpers

In addition to the standard winning opportunities, many online slots also offer additional opportunities to win. These so-called additional or special features serve, in addition to the additional winning opportunities, to maintain the game tension for a long time and thus encourage the player to continue playing the machine. In the following section, we have taken a look at which additional features are most frequently offered in slot machines.

Free Spins

A feature that can be found in many slots are free spins. As the name suggests, you get these games for free – so there is no need to make a financial commitment. Usually, these free spins are activated when you spin a certain number of scatters (which we have already described above). In most cases, three simultaneously activated scatters are necessary to start the free spins feature.

Depending on the machine, you will then be given a certain number of free spins. These free spins are then automatically activated and all winnings go to your account. Usually, free spins also offer other special features. For example, wilds can appear more often or some symbols can expand. Often there is also the possibility to win more freespins during the free spins. All the details of how the free spins work on a particular slot machine can be found in the instructions of the respective machine.

Pick and Click Games

Besides free spins, pick and click games are a popular slot feature that can provide more gaming excitement and good winnings. In a pick and click feature, you have to choose from several symbols or objects and unlock winnings through your selection. These wins are mostly cash wins or free spins. For example, in one game you may have to choose from five different treasure chests. These treasure chests will then contain cash prizes. These pick and click games usually provide very good entertainment and can provide nice extra winnings.


Jackpots are a feature that is not offered on every slot machine. Jackpots are very large wins that are activated at very irregular intervals. The chances of winning a jackpot are usually not very high – however, if you do win a jackpot, then that win can run into the millions. We do not want to go into detail about different jackpots here, but jackpots can be additional features that are activated in the game. Therefore, we would like to mention jackpots in this context.

Additional Features in the Base Game

Apart from the previously mentioned features that need to be activated separately, many slot machines also offer additional features that can be activated quite randomly in the base game. Let’s take a look at some of these features now:

Re-Spins With this feature, you get another free spin after a spin you made. This spin may have another special feature integrated. For example, a wild remains as a static symbol on the reels.

  • Drop Down Feature This feature is offered on some machines after a winning spin. Here, after each win, the winning symbols disappear and new symbols fall onto the reels and can provide another win. Usually, this feature is active until no more wins are made. Here you can earn long winning streaks with a little luck.
  • Expanding Symbols Certain symbols can expand across an entire reel or onto adjacent reels. This increases the chances of winning.
  • Symbols are replaced Random symbols can be replaced by other symbols. For example, low-value symbols are replaced by wilds.

These are just a few features that can be activated in the base game. For more details on these features, it’s best to check out the instructions of the respective slot machine.

Chances of Winning and Payout Odds

In this section we want to answer the following questions: Which slot machine game has the best chance of winning? How does a random number generator work and what about the slot machine payout ratio law? Of course, slot machines should be gambled primarily for entertainment. However, getting good winnings is also a pleasant side effect. To find out which slot machine has the best chance of winning, it is useful to look at the payout ratios of each slot machine. These payout ratios, also called RTP (Return To Player), are indicated in the game instructions for almost all slot machines.

This is a percentage that is usually between 94% and 97%. To explain the value it is good to look at an example. If a player plays a slot with a payout percentage of 95%, and makes 100 spins with a bet of 1€, he should have a balance of 95€ after the 100 spins. So this means that in the long run you will lose on a slot machine. The €5 you lose is the house edge of the casino. However, the payout percentage is only a guideline for which it takes a large number of spins to be correct. Of course, you can also collect a big win after just five spins.

Nevertheless, the payout percentage is a very good guideline to determine which slot machine algorithm is more likely to bring more winnings. So, as a rule, you should rather choose slot machines that offer a higher payout percentage. There is no general law for the amount of the payout ratio. However, casinos are also in a competitive environment and therefore payout ratios are generally quite good for online slot machines, otherwise customers will switch to other slot machines if they do not make any winnings.

So how does a slot machine determine when a win is made? For this purpose, each slot machine has a random number generator. This random generator randomly determines the symbols that appear on the reels after a spin. Therefore, it does not depend on whether you play more often, place higher bets or play the same game at a different casino. Winnings are always made randomly. Casinos have hired their own external and independent companies to test the slots’ random number generators on a regular basis to ensure fair game play. In order to obtain a casino license, it is also necessary to perform these tests. All casinos that we test at Slotsia have a casino license and the random number generators are tested regularly.

The Random Number Generator

The heart of every slot machine is the so-called Random Number Generator. This means random number generator. This generator is part of every slot machine and is activated whenever you click the spin button.

Each number of the Random Number Generator represents a symbol on the slot machine. Depending on which number combination the generator spits out, a symbol combination is played. If these symbols are on the same payline, you win. If this is not the case, it is an unsuccessful spin.

The most important point about the Random Number Generator is the fact that this system has no memory or history. This means that each result of the Random Number Generator does not depend on previous results.

Each result of the generator is therefore purely random. This can be compared to the results of a roulette game. Here, too, each game result is completely independent of the result of the previous game round.

Who guarantees that the Random Number Generator is not manipulated?

Large software manufacturers such as Netent or Microgaming invest a lot of money to ensure that the Random Number Generator is programmed correctly and without errors. These manufacturers and also online casinos themselves hire external companies to test the slot machines for correct functionality.

These third party companies perform tests of all slot machines at regular intervals. This ensures that the Random Number Generator works properly and without errors in the respective mobile casino.

What should I do if I think a slot machine is programmed incorrectly?

Although it is unlikely, it can happen that a slot machine malfunctions. If you are affected by such a malfunction, it is important that you act correctly.

First of all, you should take a screenshot of the game at the moment when the malfunction occurred. With this screenshot you should then immediately contact the respective customer service of the new online casino. Here you have the so-called log files of each spin available and the casino can understand whether it is actually a bug in the game.

As said, such a scenario is rather unlikely, but should it occur, the most important thing is that you have a screenshot of the malfunction, otherwise it may not be so easy to prove this malfunction.


Slot machines represent the largest number of games in online casinos. If you are a beginner in the field of slot machines, then hopefully we could bring you closer to how slot machines work. In principle, slots always work according to the same pattern. However, the ways to win are often different. In addition, slot machines often offer different additional features.

These include free spins, pick and click games or jackpots. As a newcomer, we recommend you to study the respective game instructions carefully before you start with a new slot machine, in order to prepare yourself for the peculiarities of the respective slot. If you take a look around Slotsia, you will also find a lot of reviews on the best slot machines online. In addition, we have tested for you all casinos that offer a huge number of slots. Just check back with us regularly so that you can always stay up to date with the latest online slots.