Random Number Generator
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. It is a Random Number Generator that chooses the reels positions. The machine's software contains it.
Random numbers are constantly generated by the RNG at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. That means that the result depends on exactly when the game is played. The result would be different a fraction of a second earlier or later.
Some professional gamblers observe that the RNG does not actually generate random numbers. The sequence of numbers is repeated by most of the RNGs. Poor programming determines such behavior. Pseudo RNGs with very long periods canbe built relatively easily. A single period can't be completed by any computer in the expected lifetime of the universe. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. It was he who discovered equations for specific gambling games like Keno. 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 see the secquence.

Payout Percentage
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. The term theoretical payout percentage is used to denote it. Jurisdictions serve to control the minimum theoretical payout percentage. It is typically established by law or regulation. A certain winning pattern is determined by every casino individually. 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. The rest of the money is given back to the players.
A slot machine's software is written and the theoretical payout percentage is set simultaneously. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. Based on current technology, this is a time-consuming process and as such is done infrequently. One can find a tamper-evident seal on the EPROM in certain jurisdictions. It can only be changed in the presence of Gaming Control Board officials. 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 can be done 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
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. A small amount is contributed by each machine in the group to this progressive jackpot which is given to a player who gets a specific combination of symbols. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
In some cases multiple machines are linked across multiple casinos. The machines are then owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is controlled by computer software. That is why it is possible to make the slot machine frequently display combinations that are close to winning combinations.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
There is a related phenomenon that is also sometimes called near-miss. 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. Only if abbreviated physical reels are used to display a win pattern based upon the RNG this can occur. In video slot machines, virtual reels are used and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also investigated by the Nevada Gaming Commission. If the near-miss above or below the pay line was not specially programmed it was stated to be legal. In other words, the near-miss must be just as likely to occur as any other combination. It is impossible to program the machine to 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.

Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by 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.
It is not as simple as that. The casino can’t just change the chip of the slot machine. It is illegal and it has to be permitted by the commission.
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.
Once again, the computer determines a win or loss and the temperature of the machine or moneyirrelevant. Although it's funny that people still burn their fingers with lighters believing this works.
Casinos set slot machines to payback more on the weekends.
One more rude mistake. The chances and odds are always the same. It doesn’t matter when you play.

Slots are one of the oldest casino entertainments. They came through a number of changes and improvement. But the objective and the principle of the game always remain the same. Basically, you just put in the coins and wait to find out whether you won or not.
Basically, slot machines are all the same. They have different odds (and of course payouts), different payoffs, different symbols and different exterior arrangement, but there's little difference under the hood. There are so many variatisn of slot machines that everybody can find something to his liking.
Two most distinct types of slot machines are single line and multiline slots. In single line slots, the machine has just one line of reels, usually with 4 or 5 wheels, while in multiline there are a number of reel lines. Multi line slots are more difficult to understand because you win according to the winning combinations on the paylines, which can be formed in many ways. Sometimes you don't win anything at all, and sometimes multiple combinations award you with relatively big winning money. There are machines with paylines; that is, vertical lines along which a winning combination must occur, but some don’t have them, as the winning combination may come in diagonals, zigzags, strange curves and in other ways and fashions.
Another interesting variation of the slot machine is a jackpot. In jackpot games, the slot machines accumulate a fraction of every wagered bet into a virtual pot. So the jackpot becomes bigger and bigger with every bet. Jackpots can reach several hundreds of thousands dollars. A certain winning combination hits the jackpot, and the lucky gambler collects everything that the ones before him left behind.
Since slot machines appeared they have been played with coins. The new invention is a slot machine online, which doesn’t differ greatly from regular slot machines. Indeed, as well as with all small wager games, there are top bets - they're known as table limits in table games. Slots are the easiest casino games. They don’t require any decision on the player’s part.
Software slot machines (as well as the digital ones) are equipped with a random number generator which is what actually plays the game. This methodic allows very fair and precise balancing without actually hurting neither the players' interests nor the casino revenue.

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. Today’s slot machines are light years away from those lumbering machines of old. These machines contain computer chips that are programmed to select numbers randomly. After selecting a number a corresponding symbol appear on the screen. 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. If inspectors approve the brand, they can produce machines, and change them. But these changes must cover legal requirements. Using their right to produce slot machines they insert different chips, which makes the machine pay less.
There is an example when there was inserted an illegal tip which doesn’t allow royal flushes. Or a chip that shows near misses of a jackpot. The effect was two-fold: the machine didn’t pay off, and customers continued to play the machine, thinking that they were very close to a big jackpot. If you think you’re playing a machine that is not honest, you can complain. 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 is a man who managed to cheat slots.
It is impossible to cheat the slot machine, but to improve the payout is possible. It was proved by a gambler whose name was Tommy Glenn Carmichael.
Carmichael, now 53, had a long history of inventing devices to cheat slot machines. His first invention worked, but not for a long period of time as he was sentenced to prison for using it. Upon his release, he continued to work on the problem and to devise a better way to cheat the new slot machines. For example, he invented a tool called a slider made from guitar wire that he used to insert into the machine and trip a switch, which released coins from the hopper.
Later the slot machines were not any longer mechanical, they were computerized and as a result there appeared a need for a new device. He fooled a manufacturer into believing he was a customer and showing him the inside of a slot machine. It was enough for Tommy. His new inventions was a light want that blinded the sensor of the slot machine and as a result got winnings. Carmichael marketed these wands to other slot cheats, making as much as ,000 a day. He drove expensive cars, took cruises, and bought two houses.
But as known good things can’t last forever. For Carmichael, they ended in front of a slot machine at Circus, where he was caught with the light wand when he tried to run from security. He was fined, but soon came back to the same business. By this time, authorities had tapped his phone and recorded conversations with other cheats who used his device. Three years’ probation and expropriation was the punishment. He has been banned from entering any casino.