Millions of gamers play online slots every day without understanding the inner workings of the machine.

Walk into any land-based or online gambling for real money casino and you’ll see the highest number of people in the slots area. Online casino and brick and mortar slot machines are the most popular games in any casino. You don’t need to understand how they work in order to maximize their entertainment value but it’s a good idea to give yourself a basic understanding of the inner workings of the game.

History of Slot Machine’s Construction

Slot gaming dates back to the late 1800s when the “one-armed bandit” was introduced to salons and other entertainment venues. These early games included 5 drums with a total of 50 playing cards. Players would insert their nickel and pull the lever to try to line up a winning poker hand. Payouts were, at first, in the form of a piece of candy or a drink at the bar. In Australia, slot machines are still called “the pokies” today based on the machine’s early connection with the game of poker.

Over time, the 5 drums were replaced with 3 reels and symbols were substituted for playing cards. Players would try to line up 3 matching symbols on the reels to achieve a payout. When it became illegal to give money for wins, the machines began to be called “fruit machines” and the symbols turned into fruit symbols. Prizes were paid out in sweets and chewing gum of the corresponding flavor. 

For many years, slot machines were purely mechanical. The player would pull down a lever on a slot machine and that would set the reels in motion. A spring inside the machine would stretch and that would eventually stop the reels from spinning.

The first fully electromechanical slot was introduced in 1964. It was called Money Honey and involved reels that were operated electrically. This was the first slot machine with a bottomless hopper and it could pay out up to 500 coins automatically. Early electromechanical slots were still launched with a lever but over time the lever gave way to a button that the player would press to start the reels turning. 

Video Slots came into being in the 1970s and proved to be very popular. Most slots today are video slots, both at online casinos and in land-based casino sites.  The video slots of the ‘70s were augmented in the ‘90s with a second screen bonus round. When the machine triggered a bonus round, a completely different screen would emerge and the player would play the bonus round on the 2nd screen. Additional payouts could be won during this bonus round. This is the accepted form of slots today.

Today slot machines are responsible for approximately 70% of a casino’s income and they also take up 70% of the available floor space in a casino.

Today’s Slot Machine Construction 

Gamers play slots because they’re fun. They are also extraordinarily profitable for the casino. They operate in a way that gives the casino a significant house edge on every spin.

Some of a slot machine’s elements include:

Random Number Generator

In the beginning, slot machines were mechanical so their random number generator was mechanical as well. Today each slot machine has its own built-in computer that generates random numbers. It is these numbers that determine the outcomes of the game.

The random number generator must create results that are truly random. Slots don’t get “hot” or “cold,” there’s no cyclical basis to a slot machine and there’s no such thing as a jackpot that’s “due” to be triggered. There’s no way to predict the outcome of a spin any more than you can predict what the next card will be when dealing a deck of cards.


Today’s slots usually have 5 reels. The reel is the image that spins in front of the machine. There are symbols on the reel which are generally based on the game theme. When you line up a specific combination of symbols, you win the payout that’s associated with that particular symbol. Reels used to be metal hoops but now they’re images on a video screen. Remember, the outcome of any spin is determined by the random number generator which is located within the computer that powers the screen.


When the reels stop -- on a symbol or on a blank space between symbols – it’s called a “stop.”  The odds of a symbol coming up differ, based on the symbol.  Some symbols come up on an average of once every 5 spins while others – the high paying ones – come up an average of once every 30, 40, or 50 spins. 

Weighting System

The more stops on a reel, the easier it is for the casino to offer large jackpots. If, for instance, there’s a game with 10 stops on each reel and there’s an equal chance of landing on each stop, the chances of the player winning any particular combination are 1/10 X 1/10 X 1/10, or 1/1000. If the payout is over 1000 units, the casino would lose money on that game. Therefore, modern slots are powered by computers that can adjust those odds using a weighting system.

The weighting determines how likely a particular stop is to be pickedOn a slot machine game with 10 symbols and one “special” symbol that only comes up every 100 spins, the odds that the player will get three of the “special” symbol are 1/100 X 1/100 X 1/100, or 1/1,000,000. The casino could offer a payout of $1 million on that combination and still break even. Of course, casinos love that kind of action but, interestingly enough, players like to test their luck with those types of odds as well.

Par Sheet

The par sheet is the sheet that specifies the weightings for each stop on the reel and thus determines the odds. The par sheet makes the house edge and the odds for a slot machine game a known quantity. Casinos receive the par sheet for each slot machine but those par sheets are never made public so the gamer never really gets a clear idea of what the house edge, the payback percentage or the odds involve.

Payback Percentage

The payback percentage is a mathematical prediction of the amount of money the machine will “payback” over an infinite number of spins. The machine is programmed to have a specific payout rate – on a payback percentage of 97%, for instance, out of every $100 put into the machine, $97 will be paid back to players.  The remaining $3 goes to pay the casino's costs and profit.

97% payback at online casinos is quite average.  Land-based casinos have a much lower payback - around 60% - as their costs are much higher than an online casino.  Casinos distinguish between the actual return on the machine and the theoretical payback percentage. 

If slots are your preferred form of casino entertainment, by all means, play. But you’ll have a more informed gaming experience if you know exactly how those slots work.