How do Slot Machines work?

Is a casino even a casino without slot machines? These machines have changed the world of gambling since their introduction in the late 1800s, and even been the foundation for the growth of cities like Las Vegas. But how do they actually work?

CasinoPlay went deep into the inner working of slot machines, past and present, to give you an exclusive insight into how these popular machines work.

How do slots work?

It’s super simple. You pull the handle or push the button. The reels spin. The reels stop. You win or lose.

Simple yes but also no. How they do what they do and why they do what they do is part of the mystery behind slots.

First, let’s get some terms out of the way:

Coin slot

This is what slot machines are named after – the slot that you put your coin in to start the game. A lot of modern slots, and online slots especially, have done away with the need for coins and you play completely digitally with your bankroll ‘deposited’ directly into the machine.


Also called a handle, this is the old-timey mechanism for starting the reels spinning. You pulled down on the handle and a spring inside the machine started the reels spinning. Modern land-based casinos and online casinos have a ‘spin’ button.


Reels are the drums with symbols on that you can see spinning and stopping through the window or on the screen. \In old slot machines they were physical drums with the symbols printed onto them. Modern video slots and online slots use graphics and software to simulate the action and appearance of reels.

Pay line

The pay line is the ‘path’ that the symbols need to line up on in order to trigger a win. The first 3-reel slots had a single pay line that run across the middle of the window. If the symbols lined up along this line, the player won. Modern slots games can have thousands of pay lines. Megaways slots games can offer up to 586,971 ways to win!

Pay lines are not always straight lines. They can be straight or diagonal or geometric patterns and more. They can run from left to right or from right to left. The pay table will show you exactly how the pay lines work, and modern slots take the guesswork out of the equation by automatically reading the lines and calculating your win.

Some slots games have fixed pay lines while others have dynamic pay lines that move to create the patterns.

Pay table

A pay table is a list of the winning combinations and amounts offered by the game. In land-based casinos, the pay table is usually printed just below the screen. In online casinos, the pay table is a pop-up window on the online slots game that you can click to reveal.

Slot reels explained

The reels are the areas of the slot machine that have the symbols. You can have three, five or seven reels on any slot machine or game.

The reels are set in motion when you pull the handle or hit the spin button. They spin and stop at random intervals to reveal the symbols that are in play. Depending on how the pat lines are set up, these symbols will form a pattern (a straight line is the most basic pattern, while various combinations or geometric shapes can be available in more modern and complex slot games). If the symbols line up with the pattern, the player wins. If not, the player spins again.

Reels will have different symbols depending on the theme of the game. The most recognisable (from the history of slots and the reason why they are also called fruit machines) are the fruit symbols: oranges, lemons, cherries and plums. Card-based symbols are also well used: diamonds, hearts, clubs and spades. Modern online and land-based slots games come in thousands of different themes like blockbuster movies or ancient history or fantasy and their symbols will reflect each theme.

What are 3-reel slots?

3-reel slots are land-based or online slots games that have three independent reels. These were among the first examples of slot machines that were developed and helped to usher slot machines into the global consciousness.

3-reel slots fell out of favour to a small extent with the development of the technologically more advanced 5-reel slots. However, there is a growing return to popularity for this format as they are fast and fun to play and are now being updated to offer all the bonus games and bonus features of their bigger cousins.

What are 5-reel slots?

5-reel slots feature five independent spinning reels. These are by far the most popular forms of slots games in land-based and online casinos and have been a major focus for games developers. This has seen the 5-reel slots games offering better graphics, better sound, more themes and bigger bonus features than the 3-reel games, although this is changing.

What is a Random Number Generator?

In the old days, a slot machine’s reels were connected to a spring, a shaft and a handle. When the handle was pulled, the shaft compressed and released a spring which made the reels spin. Gravity, inertia and various other scientific principles then came into play as the reels slowly stopped and the symbols came to rest and revealed the final combination.

Modern slots machines, and especially online slots games, rely on technology and software to create the sense of spinning reels but it is actually all a computer simulation. What makes modern slots a game of chance rather than a preprogramed outcome is the Random Number Generator or RNG.

The RNG is a software programme that ensures that the final outcome, the places where the reels stop, is completely random.

RNGs are dedicated hardware devices that continually generate random numbers. When you hit spin on the slots game, an RNG generates a random number, feeds it to the mathematical module and this determines where and when the reels will stop. This means that there is absolutely no way you can predict the outcome. If you hesitate for a split second before hitting spin you will have a completely different random number than if you hit it immediately. Playing online slots games or land-based slots games is truly all about luck.

Payout percentages

Payout percentages or Return To Player (RTP) are mathematical statistics on how often a particular slots game will pay out over time. The higher the RTP, the more likely you are to hit a winning spin.

RTP does not guarantee you will win. It’s more a calculation based on how much money is gambled over how much money gets paid out. For example, an RTP of 98% (The average for a good slots game) means that OVER TIME the game will pay out 98% of the money wagered. If you bet R100, then you could expect to get R98 out – in the long term.

Or you could be lucky, hit the jackpot and retire to Bora Bora.


Games developers and casinos offer a host of bonuses to get players to try out their games. Online casinos in particular are famous for offering great incentives to new players.

Bonuses in the slots game world are a bit different.

When speaking of slots games bonuses, we are usually referring to all the add-ons and extras that a game offers. Early slot machines gave you the chance to pull a lever, watch the reels spin and then check a single line to see if you were a winner.

Modern slots are fully immersive entertainment destinations that technology has allowed to offer way more.

Bonuses can include:


  • Symbols that take the place of any other symbol to create a winning combo.


  • Scatters are special symbols that unlock bonus games or free spins

Free spins

  • Free spins are spin you win that don’t cost you money.

Bonus games

  • A lot of modern slots games have mini bonus games like pattern matching or spin and win or lucky card games that allow you to win extra money, spins or prizes.

Cascading symbols

  • Cascading symbols work like a game of Tetris. If you hit a winning combo your win is banked and the symbols disappear to be replaced by new symbols and potentially new winning combos which starts the process of all over again.


  • Multipliers are special game mechanics that give you the chance to multiply your win exponentially.

Progressive slots

Progressive slots are the Big Daddy’s of the slots world. These are the games that can pay out truly life changing amounts (in the multiple millions of dollars range). It is, however, extremely difficult to hit that massive jackpot and you have to play with max bet which can make it very costly too.

Progressive slots are a whole series of games linked across casinos and countries where a small percentage of every bet from every player goes into one huge and constantly growing jackpot.


There are a ton of websites and people who will try to sell you on a fool-proof way of swinging the odds on a slots game in your favour. CasinoPlay are here to tell you straight out: there is no way.

With modern RNG hardware and encrypted technology and lightning fast software there is no way to work out the probability of winning on a slots game. You can use variance and RTP to give yourself an idea of how much you could potentially pay while playing, but never to work out how to win at slots. For more information on variance and RTP, see our ‘How to win at slots’ page.

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