First slots machines were mechanical devices and the odds of such machine were determined by the number of reels and symbols on each reel. The selection principle was an independent spinning of each reel.
The Liberty Bell had three reels each with ten symbols and every symbol on the reel had an equal probability of showing on the payline. The chance that a symbol appear on the paying was 1 in 10. So the odds against hitting the jackpot on the Liberty Bell were 1,000 to 1, because account for 10 x 10 x 10 = 1,000 possible combinations.
In the new computer controlled slot machines this is not the case as the reels have no independent action. They are being perfectly coordinated by the RNG and the computer to achieve a certain payback percentage.
The first thing you should realize about slots odds - every spin is random and is in no way related to the last or to the next spin. Every time you pull the handle a new random number is generated. And only this number determines the outcome. The ideas about "slot machines cycles”, "cold and hot machines" refer to the slot machine myths.
Another important notice - in the modern slots, whether they are three reels, video machine or online slots - not every combination shown on the payoff table has the same chance of being hit. It is determined by the computer chip - the lower wins will be hit many more times than the top prize to achieve the certain payback percentage. So, while every spin is totally random, every combination is not equal.
There is a popular opinion, that the casino can "tighten up" a machine. It is not the case. The odds for a particular slot machine are built into the program on the machine's computer chip. Usually, the casino cannot change the odds on a machine without replacing this chip.
In most gambling jurisdictions, the law requires that payback percentages be above a certain level, nowadays a slot machine returns between 85 and 100 percent, depending on the game and location and the average house advantage is calculated to be about nine percent. It means, that in the long run the particular machine will give back 91 cents for every dollar played. It keeps nine cents on the dollar.
Of course, people would not play slots if every time they put a dollar in they got 91 cents back. Instead, a slot machine is programmed to return percentages explosively and this volatility is what makes slot playing fun - you have exactly the same chance of hitting the jackpot every single time you pull the handle.
You will hardly find other casino game with such an enormous built-in edge against you. Nevertheless, slots are so popular today that they now constitute 70 percent of all casino action.
Of course, the house edge reduces your chances of winning the jackpot as compared to mechanical slots. On the other side, if you hit the jackpot in the modern slots it will be much larger then the original ten nickels of Liberty Bell.