Slot machine

A slot machine or simply slot, is a casino machine with three or more reels which spin when a button is pushed. Slot machines are also known as one-armed bandits because they were originally operated by one lever on the side of the machine. Many modern machines are still equipped with a legacy lever in addition to the button.The machine pays off based on patterns of symbols visible on the front of the machine when it stops. Modern computer technology has resulted in variations on the slot machine concept. Slot machines are the most popular gambling method in online casino. The games are often produced by games producers like Netent, Microgaming, Quickspin,Yggdrasil and Play’n Go.

In an online casino these slot machines are usually grouped as classic slots or video slots, where classic slots are online iterations of brick-and-mortar casino games while video slots are much more advanced and hold features that a physical machine wouldn’t be able to.

Some slot machines also offer a jackpot as the biggest win, these are commonly named jackpot slots.

Video slot machines

The video slot machine is a digital version of the mechanical slot machines, and is essentially like a video game. This allows game producers to be able to offer more interactive elements, such as advanced bonus games and advanced video graphics.

Also, because there’s no mechanical parts involved in the designs – most video slot machines have more reels than three. This increases the possibilities for game designers, being able to use as many as 50 different symbols on one reel, giving odds as high as 300 million to 1 – which is enough for even the largest jackpots. Since there’s so many combinations with five or more reels, the game producers do not need to weigh the payout symbols (although some may still do so). Instead, higher paying symbols will typically appear only once or twice on each reel, while more common symbols, earning a more frequent payout, will appear many times.

Video slot machines typically also lets the player play multiple “lines”, so instead of just taking the middle of three symbols displayed on each reel – a bet line could go from top left to bottom right, or any of the other patterns specified by the manufacturer. As each symbol is equally likely, there is no difficulty for the manufacturer in allowing the player to take any or all of the possible lines on offer – the long-term return to player will be the same. The difference for the player is that the more lines he plays the more likely he is to get paid on a given spin – though of course he is betting more in the first place.


The “columns” that show the game symbols in a slot machine are referred to as reels. Historically, all slot machines used revolving mechanical reels to display and determine results. Although the original slot machine used five reels, simpler, and therefore more reliable, three reel machines quickly became the standard.

The problem with three reel machines is that the number of combinations is only cubic– the original slot machine with three physical reels and 10 symbols on each reel had only 103 = 1,000 possible combinations. This limited the manufacturer’s ability to offer large jackpots, since even the rarest event had a likelihood of 0.1%. The maximum theoretical payout, assuming 100% return to player would be 1000 times the bet, but that would leave no room for other pays, making the machine very high risk, and also quite boring.

In the 1980s, however, slot machine manufacturers incorporated electronics into their products and programmed them to weight particular symbols. Thus the odds of losing symbols appearing on the pay line became disproportionate to their actual frequency on the physical reel. A symbol would only appear once on the reel displayed to the player, but could in fact occupy several stops on the multiple reel.


Refers to the characters on the reels, which is matched into pay lines to form winning combinations. There’s a few basic variations to these symbols

Basic symbols

Are the lowest value symbols in a game, and form the lower paying bet lines. There are usually 7-10 variations of basic symbols, where the lowest five often, but not always portrayed as 10 – J – Q – K – A, referring to the card deck. The top paying basic symbols are always game-specific graphics, but comes without special features.

Bonus symbols

The symbols on the reels of a slot machine that activates the bonus game/feature.

Scatter symbols

Symbols which can create winning combinations without being on active pay lines.

Special symbols

Are symbols apart from bonus, scatter and wild symbols are specific to a certain game and comes with extra features.

  • Big symbols – A symbol that covers several symbol positions.
  • Stacked symbols – Stacked symbols refers to when two or more symbols appear on top of each other on the same reel. This can occur both deliberate in form of a special feature or at random.

Wild symbols

A wild is like the joker in a deck of cards, it is a symbol which can act as any other basic symbol to complete winning combinations. Some slot machines also allow the wild to act as a bonus/scatter symbol.

  • Cloning wild – Is a wild that clones symbols in adjacent reels to make a longer winning pay line.
  • Expanding wild – Some games include a special feature where a wild expands to fill connected spaces on the reels.
  • Sticky wild – Is a wild that sticks to its position over the next spin, or other pre-set rule. Some games award a re-spin with the wild sticking to its position. Other games keeps re-spinning as long as a new sticky wild appears, or a winning pay line is found.
  • Walking wild – A special feature popularized by the NetEnt slot machine ‘Jack and the Beanstalk’. Refers to when a Wild symbol triggers a re-spin and during the re-spin the wild symbol moves one reel to trigger a new re-spin.

Game features

Although most games have very specific features tied into the theme of the slot machine, there’s some features or mechanics that are common between developers and individual games.

Basic features

  • Bonus Feature – The extra game within the game, where you play for free with free spins. Unlocked from getting the right amount of bonus symbols shown on the reels.
  • Coins – Coins are the currency within a slot machine. Value of coins can be adjusted with the coin value function within a game, while the amount of coins wagered are adjusted by the bet level function.
  • Free Spins
    a) A free spin within a game usually refers to the spins that a player is awarded as part of a bonus feature or mini game. They are free to play as part of the win. These free spins are activated when a bonus game is triggered from bonus or scatter symbols.
    b) It can also refer to the free spins offered by the casino on a specific game to test the basic game.
  • Level/bet level – This function to adjust how many coins a player is wagering per spin.
  • Multipliers – A feature that multiplies your win by a given number. A x2 multiplier doubles your winnings.

Special features

  • Avalanche – A feature that means winning symbols are removed and the remaining symbols “fall down”. The reels are re-filled with new symbols from the top without a new spin being initiated. An avalanche usually come with a multiplier on wins occurring on consecutively triggered avalanches – as made popular by the NetEnt slot Gonzo’s Quest.
  • Cloned reels – When triggered the reel clones itself onto the next reel so that both reels carry the same symbols.
  • Pick and click – A type of mini-game bonus round where players are prompted with a certain number of objects in line with the slot’s theme and the player clicks on whichever he/she wishes and is shown the prize or stopper. Some pick and click rounds offer the player multiple prizes, others just one.
  • Re-Spin – Is a common name for a single free spin, sort of like a do-over.

Random number generators

All modern machines are designed using pseudo random number generators (“PRNGs”), which are constantly generating a sequence of simulated random numbers, at a rate of hundreds or perhaps thousands per second. As soon as the “Play” button is pressed, the most recent random number is used to determine the result. This means that the result varies depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.

Payout percentage

Slot machines are typically programmed to pay out as winnings 82% to 98% of the money that is wagered by players. This is known as the “theoretical payout percentage” or RTP, “return to player”. The minimum theoretical payout percentage varies among games, providers and – in some cases location. The winning patterns on slot machines – the amounts they pay and the frequencies of those payouts – are carefully selected to yield a certain fraction of the money played to the “house” (the operator of the slot machine), while returning the rest to the players during play. Suppose that a certain slot machine costs $1 per spin and has a return to player (RTP) of 95%. It can be calculated that over a sufficiently long period, such as 1,000,000 spins, that the machine will return an average of $950,000 to its players, who have inserted $1,000,000 during that time. In this (simplified) example, the slot machine is said to pay out 95%. The operator keeps the remaining $50,000.

Historically, many casinos, both online and offline, have been unwilling to publish individual game RTP figures, but since the turn of the century some information regarding these figures has started to come into the public through more and more game providers and casinos releasing the information, or through studies by independent gambling authorities.

The return to player is not the only statistic that is of interest. The probabilities of every payout on the pay table is also critical. For example, consider a hypothetical slot machine with a dozen different values on the pay table. However, the probabilities of getting all the payouts are zero except the largest one. If the payout is 4,000 times the input amount, and it happens every 4,000 times on average, the return to player is exactly 100%, but the game would be dull to play. Also, most people would not win anything, and having entries on the paytable that have a return of zero would be deceptive. As these individual probabilities are closely guarded secrets, it is possible that the advertised machines with high return to player simply increase the probabilities of these jackpots. The casino could legally place machines of a similar style payout and advertise that some machines have 100% return to player. The added advantage is that these large jackpots increase the excitement of the other players.


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