The RNG
Nowadays slot machines are computerized. So the odds are whatever they are programmed to be. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. The positions of the reels are chosen by a Random Number Generator. There is a Random Number Generator in the machine's software.
The RNG generates random numbers at an extremely high rate. The most recent random number is used to determine the result when the Play button is pressed. That means that the result depends on exactly when the game is played. The result is different every fraction of a second.
Some professional gamblers observe that the RNG does not actually generate random numbers. It turned out that The number sequence is repeated time after time. The cause of it is in poor programming. Pseudo RNGs with very long periods canbe built relatively easily. No computer can complete a single period in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. Specific gambling games equations were discovered by him. The fact that the next set of selected numbers would be based on the previous games played was revealed. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So the player cannot tell where in the sequence they are.

Payout Percentage
The percentage of winnings paid out of the money that is wagered by players is 82–98 per cent. This is known as the theoretical payout percentage. The minimum theoretical payout percentage is controlled by jurisdictions. Law or regulation are used to establish it. A certain winning pattern exists nearly in every casino. It concerns the amounts they pay and the frequencies of these pay-outs. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. As for the rest of the money it goes to the players.
At the factory when the software is written a slot machine's theoretical payout percentage is set. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. As it is a time-consuming process it is done infrequently. There is sometimes a tamper-evident seal on the EPROM. Gaming Control Board officials should be called for changing it. According to other jurisdictions slot machines are randomly audited to ensure that they contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. The machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made after the change is made.

Machines That Are Linked
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or jackpot. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. A single slot machine's jackpot is considerably less than the progressive jackpot.
There can also be multiple casinos consisting of multiple machines. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss Programming
The reel display of modern slot machines is under computer software's control. It is possible to make the slot machine frequently display combinations that are close to winning combinations.
This practice is called near-miss programming.
This term is also used for a related phenomenon. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. The combinations above and below the payline are all roughly equally randomly distributed. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. The symbols that appear around the winning line in video slots are an accurate depiction of how the reels were mathematically modeled.
The Nevada Gaming Commission investigated the issue of a near-miss above or below the pay line. So long as the near-miss above or below the pay line was not specially programmed it was recognized legal. In other words, any other combination must be just as likely to occur as the near-miss. The machine can't show winning combinations more frequently than other combinations above or below the pay line.
In Australia near-miss programming, where a near miss is inaccurately displayed is not allowed either. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Slot Machines Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. An example of modern ways of slot machine defrauding is directing microwaves toward it to disrupt its proper functioning.

Here is the list of the most amusing myths concerning slots.
You can spot a winning slot machine.
The only way you'll spot a winning slot machine is when the bells are ringing and the money is dropping. RNG is the main element of every slot machine and one never knows what combination it will form.
Casinos place loose slots near the cashier, aisle ways, etc.
If this were true, it would be the only machines anyone would play. A casino mixes slots all over the casino floor. No one with any experience at playing slots would make a statement like this.
Count the number of symbols on the reels to determine your odds.
This is mathematically impossible, remember the RNG (random number generator) determines the stops. Thereare literally millions of combinations on a 3 reel slot machine, so there is no way of determining odds by the symbols on the reels alone.
Casinos can tighten or loosen slot machines from the back office.
In order to change how a slot willpay out is by changing the computer chip. No casino can just flip a switch.
A slot machine that has not been paying is due to hit.
Absolutely false. The RNG assures this. You have an equal chance of winning on a losing machine as you do on a machine that just paid out the jackpot.
Pulling the slot machine handle produces more wins than pressing the spin button and visa versa.
Again false. It makes no difference, as the computer only uses these as mechanisms to start the game. Only the computer determines a win or a loss and it has already determined if you are a winner or loser even before the first reel stops spinning.
Playing with hot coins improves your chances.
It is false. Nothing can affect the outcome, the machine doesn’t care and doesn’t know whether your coins are cold or hot.
Casinos set slot machines to payback more on the weekends.
Wrong again. Your odds of winning are the same no matter if it's 4:00pm Monday, 8:00pm Saturday or 3:00am Wednesday. Casinos can not and do not do this.

Knowing the rules is of great importance when playing any casino game. Slots are the easiest casino games. The rules are very simple. The player has to insert coins in the slot machine, then push the button or pull the handle and wait for the combination of symbols. If the bells ring and lights flash, open up your pocketbook because you are in the money! Enjoy, but never gamble with more money than you can afford to lose.
Rules for playing Slots
Only slots have so simple rules. In fact it is so because there are no rules at all. The principle of the game is the following: the player inserts coins, pulls the handle and then checks the combination of symbols on the lines. The mission of the game is to match preselected symbols on the pay line, (or paylines, depending on the machine being played, there can be up to 5 pay lines on some machines), to earn credits. Slot machines can be found in many denominations from 5 cents to being the most popular, although there are also machines that accept 0 tokens.
It is a popular game for beginners because it is just them against the machine. They don't have to worry about rules and regulations, or being laughed at by the other players for making a bad play, all they have to do is try to hit the jackpot by matching the highest paying symbols on one of the pay lines.

It is true that slots are designed to be honest and not to deceive a player. However, in the past certain manufacturers have been caught using computer chips that cheat the customer.
In the old days, slot machines worked mechanically, with independent wheels spinning until they each stopped randomly. Modern slot machines differ form the old ones greatly. There is a chip inside of every slot machine, this chip is programmed to select numbers randomly. The selected number determines the symbol which then comes out. The computer code that generates the random numbers is responsible for whether the machine is “loose” or “tight,” so gaming inspectors look at the machine’s code to make sure that a slot machine doesn’t cheat. The machine has to pay out either to the legal rate or better. Of course the latter happens very seldom.
Because slot machines don’t pay off with every spin, it’s difficult for customers to tell whether a slot machine’s chip has been programmed to pay off less often than is legally required. Before the machines are installed in casinos they are examined by inspectors. But you must understand that inspectors can’t examine every machine. They examine the brands. Once inspectors approve a brand of machine, the manufacturers can make changes to individual machines as long as the changes fall within the legal bounds. Some dishonest manufactures use the right to produce machines, but use a different chip, which is programmed to pay less or not so often.
In one of the casinos the chip was programmed in such a way that royal flushes would never come out. Or a chip that shows near misses of a jackpot. The player would never hit a jackpot but keep on playing maximum bets. Don’t keep silent if you come across such slot machine. Be sure that inspectors will examine this very machine and the casino itself to make sure everything is ok. Manufacturers that cheat are fined or they can lose their license.

There are many types of slot machine configurations but all work according to the same principles - put the money in the slots and pull the handle! The main classification of slots distinguishes two types of slots: straight and progressive slots. The main feature, which distinguishes straight slots from other types is that you get paid according to the payout table, which is shown on the slot machine. Inside the straight slot group four more types are distinguished. They are multiplier, bonus multiplier, multiple payline and buy-a-pay slots. The bet amount determines the sum of the payout in multiplier slots. Bonus multiplier slots are characterized by a bonus round, which is activated if making maximum bets. The principle of multiple payline slots is that the number of enabled payline depends on the number of coins bet. The Buy-a-Pay slots are profitable only if you place maximum bets. The jackpot for Progressive slots is never the same as it grows higher with every coin put in the machine. The jackpot total is posted above the machine and can grow to enormous sums of money.