Slot machines have undergone dramatic improvements over the past century, with mechanical slots being almost completely replaced by fully computerized machines. When slots first appeared, their principle of operation was entirely mechanical. The machines used levers, springs, and gears to work.
Then, a new generation of electromechanical games emerged. They used solenoids, motors, and electromagnets in addition to the original moving components, significantly improving the gameplay. But game manufacturers did not stop there. Today’s slots are fully computerized and rely on components like computer chips and Random Number Generators (RNGs).
Differences between Mechanical and Modern Video Slots
The theoretical payback percentages in early machines were based on the number of symbols each spinning reel contained coupled with the payout for each winning combination.
The introduction of the RNG technology in the early 1980s completely changed the way slots play.
The machines were rather bulky. They contained huge spinning reels with symbols that were set in motion by the pull of a lever. The reel number was usually limited to three because the addition of more rotating drums made the machines very difficult to install.
Traditional mechanical slots eventually gave way to modern machines that rely on computer chips and software instead of gears and springs. Modern games have no mechanical parts whatsoever.
A coin detector registered the number of coins players inserted into the machine along with their denomination. By pulling the lever, the player triggered a spring-based mechanism that consisted of three components, called a hook, a kicker, and a control cam.
These components operate together to release the drums from their resting position and determine how smoothly they rotate and with what force. Other mechanical components involved in the spinning of the reels include gears, notched disks, and stoppers.
They graphically simulate the rotation of the reels on a video screen after the player presses the spin button. The outcome of each round is determined by a microprocessor inside the video slot.
This enables game developers to incorporate more symbols and reels into the games, which, in turn, increases the number of possible combinations. Interestingly, video slots did not immediately become a hit on the casino floor.
Long strips containing symbols are glued on the reels with adhesive. Next to all symbols, there are notches called stops. The higher the number of stops associated with a given symbol, the greater the chances that it appears on a payline. Jackpot symbols usually have a single stop while blanks have two or more stops. Because of this, blanks hit far more frequently than symbols that pay out.
The reels come to a halt one at a time, which creates more suspense. The player is awarded a payout when all three reels contain identical symbols. However, the matches must occur on a payline for the machine to yield a payout.
Many players were wary of the new games because they had no trust in computers. The imaging was of low resolution and the slots lacked immersive sounds and bonus games to keep players interested.
This all changed in the mid-1990s when the first bonus-game slots hit the casino floors. Manufacturers like the Australian company Aristocrat Technologies launched slots that played on five reels and had bonus features like free spins. Players gradually grew warmer toward video slots, with Reel 'Em In by WMS becoming a massive hit in the United States in 1997.
Computerized Slot Machines
Modern slots are software-driven, which enables developers to incorporate more symbols, high-definition graphics, animations, and special features into the gameplay. There are several key components behind video slots, starting with the Random Number Generators (RNGs) that determine whether a spin wins or loses. Virtual reels are utilized, allowing for a higher number of paylines and more complex paytables.
Random Number Generators in Slots 
The Virtual Reels 
The Payline 
The Paytable Random Number Generators in Slots
The RNG is at the heart of a video slot, ensuring that each spin has an equal chance of producing a winning combination. The RNG is capable of generating hundreds of random numbers each second and uses complex algorithms for this purpose. It is always working even when no one plays the machine.
As long as the slot is plugged in, the RNG is spewing random whole numbers, usually between 1 and a couple of billion. The computer records the numbers the RNG has generated in the exact millisecond when the player has hit the spin button and feeds them into a program to decide where the virtual reels must stop.
All combinations are equally possible on each spin because the RNG uses a full set of numbers on each round. A random number is selected and mapped on each of the spinning reels, telling the game in which position they must stop. The reels are set into monition to help build suspense but this is just for show. The outcome of the round is predetermined the moment the player hits the spin button.
The way the symbols are mapped on the virtual reel is game-specific. In some of the more advanced games, pressing the Bet One button, the Maximum Bet button or pushing the lever causes the algorithm to start at different points.
The use of computer chips has revolutionized slot machines, giving manufacturers and players alike more flexibility. You can now use bills to purchase a predetermined amount of credits instead of feeding coins into the machine on every spin. Additionally, both players and gambling operators can easily track what amounts are won or lost on a given machine.
The Virtual Reels
Modern slots use virtual reels, which allow for a much larger number of possible symbol combinations than that in games with physical reels. After all, mechanical reels must fit into the slot’s casing. The addition of more symbols per reel is possible but only up to a certain point.
Casinos need more symbol combinations than mechanical reels can offer if they want to provide slots with odds capable of producing huge jackpots. To achieve this, symbols and spaces are mapped on the virtual reels, which significantly increases the number of potential combinations.
Suppose you have a slot where the physical reel has 10 spaces and 10 symbols, for example, 1 lucky seven, 2 plums, 3 lemons, and 4 cherries. Depending on who manufactures the slot, each identical symbol can be given an equal or a different number of stops.
In the example below, alike symbols have an identical number of stops on the reel – 1 for the lucky seven, 3 for the plums, 5 for the lemons and 7 for the cherries.
| Symbol | Random Number Producing the Symbol | Overall Number of Stops for the Symbol |
|---|---|---|
| Lucky Seven | 1 | 1 |
| Plum 1 | 2, 3, 4 | 3 |
| Plum 2 | 5, 6, 7 | 3 |
| Lemon 1 | 8, 9, 10, 11, 12 | 5 |
| Lemon 2 | 13, 14, 15, 16, 17 | 5 |
| Lemon 3 | 18, 19, 20, 21, 22 | 5 |
| Cherry 1 | 23, 24, 25, 26, 27, 28, 29 | 7 |
| Cherry 2 | 30, 31, 32, 33, 34, 35, 36 | 7 |
| Cherry 3 | 37, 38, 39, 40, 41, 42, 43 | 7 |
| Cherry 4 | 44, 45, 46, 47, 48, 49, 50 | 7 |
| All Symbols | 1 through 50 | A total of 50 stops |
Of course, it is also possible to design the game in such a way so that there are 4 random numbers for lemon 1 on the first reel, 5 numbers on the second reel, and 6 numbers on the third one, if this yielded the odds you were looking for. It all comes down to what odds you want to give to the game.
In today’s slots with mechanical reels, the blank spaces between the symbols are stops, too. We used only symbols in our example for simplicity. The probability of winning the top-paying combination of three lucky sevens is 1 in 1,000. Respectively, one such game will not yield a huge jackpot even if the designer significantly decreased the payouts of other matching combinations.
When symbols are mapped on virtual reels, the probability of winning with three lucky sevens dramatically decreases to 1 in 125,000.
This enables the game to yield top prizes that reach thousands of credits while leaving room for more payouts with other matching symbols. Thanks to symbol mapping and virtual reels, slot developers are now capable of designing slots with lower variance. This is possible by decreasing the game’s jackpot and increasing the payout of low-paying winning combinations.
The Payline
The payline number is easily one of the most important features to consider when choosing what slot to play. A payline or a win line is a pattern of matching symbols on the reels which awards a payout. The paylines can run across the middle of the reels, diagonally, in zig-zagging patterns and so on.
Classic slots with three reels have a limited number of paylines while their advanced video cousins boast multiple lines whose number can reach 100 in some games. In slots with five reels, there is no need to hit identical symbols on every single reel to win.
Some video slots award payouts for aligning three or even two matching icons on a payline. The more matches you score, the bigger payouts you get. In some games, top-paying symbols return smaller prizes for two identical symbols only.
More often than not, the rules stipulate the symbols must occur from the leftmost to the rightmost side of the reels. You normally receive nothing for four cherries on the second, third, fourth, and fifth reels if the symbol is absent from the first reel. The paylines in some games pay in both directions. You collect a monetary prize as long as you have like symbols align on any of the paylines.
Playing multi-line slots is preferable becomes you can win with several paylines on the same spin. The more paylines there are, the greater chance you have to collect a prize. With that said, it is essential to clarify that you need to place a bet on a win line to activate it. Otherwise, you will not pocket anything even if identical symbols land on the payline.
The number of paylines is fixed in some slots. Players cannot activate or deactivate lines but have to cover them all with one or more coins on each spin. Suppose you play with 20 fixed paylines where you can bet between 1 and 5 credits per line. The minimum bet you can place will amount to €0.20 per spin, provided that you play at the first level, betting 1 coin per line with a denomination of €0.01.
In other games, you can adjust the number of paylines you play per spin by deactivating some of the available lines. This is not necessarily the wisest course of action because winning combinations do not pay when they land on deactivated paylines.
The Paytable
The paytable is where reel spinners will find all the important information about the slot they are interested in. It contains all winning combinations of symbol alongside the number of credits they can earn you. Additionally, the paytable lists the bonus features incorporated into the slot and the conditions players must meet to trigger them.
In most cases, the listed coin payout values are based only on the first betting level where you wager one coin per line. You usually can open the paytable by clicking or tapping the info button on the main game screen.
Below is an example with the paytable of NetEnt’s Starburst slot where you need a minimum of three identical symbols on one of the 10 paylines to win a prize. The game has five betting levels, allowing you to wager up to five coins per active payline. As paylines cannot be deactivated here, a single spin will cost you a minimum of €0.10 (€0.01 x 10 paylines).
If you inspect Starburst’s paytable closely, you will notice that some winning combinations do not fully cover the cost you paid for the spin. Such is the case with the purple and blue gemstones that return a minimum payout of 5 credits for 3 matches at the first betting level.
As you can see from the image, we won with a €0.20 bet of two coins per line (2 coins x €0.01 x 10 lines = €0.20). A combination of 3 blue gems returns 5 credits per coin wagered. Since we scored wins on paylines 1, 3, 5, 7, and 8 at the second betting level, the spin resulted in profits of 5 x €0.05 x 2 coins = €0.50. Not the most impressive win but it still suffices to show you how multiple paylines and bet levels work.
Starburst has a more basic paytable than those available in advanced slots. Some games offer a higher winning potential, with more paylines, symbols, and winning combinations. Many slots use additional playing card symbols (usually 10, J, Q, K, and A) for their low-paying combos.
Starburst has only one special feature (up to 3 free re-spins) although there are games with multiple features like free spins and bonus games. The maximum credits you can wager per spin is 100 coins but this limit can be significantly higher in other slots where players can risk 400 and even 500 coins.
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