How Slot Machines work

Slot machines happen to be quite simple. A RNG picks out random numbers that get mapped onto the reels' symbols. With three-reel slots, independent reels are usually used, which means that the machine will choose a different number for every reel. Conversely, when five-reel slots are used, dependent reels are usually used, which means that the machine will choose five reels all at once, as a set.

Three-Reel Slots

When it comes to three-reel slots, the RNG will pick three numbers at random, each number of which will stop on every reel. After that, the machine will direct the reels and make them stop on selected spots that the RNG chooses. This principle holds true for five-line slots, as well: five numbers will be picked out at random - one for every reel.

Keep in mind that the game ends after the reels start spinning. By then, the RNG already has the stops all picked out, and all of the reels will merely spin as pure courtesy for the player. The truth is: reels aren't actually needed in slot machines. You could simply throw money into the machine and immediately find out if you are a loser or winner. The physical reels actually being there don't make any difference during the game; they are merely there to show the player what was picked out by the computer.

If you ever get to see the inside of a machine, you may see an unfolded reel where different symbols are all spread out across 22 different stops and any blanks are stops, too. You may think that because 11 blanks exist, there are 50% odds of getting one. Plus, because only a single jackpot symbol exists, your chance of winning the jackpot is 1 out of 22. However, this isn't how things work because 22-stop reels aren't always used. The invisible reel actually has at least 128 stops, which the computer controls. A number will be chosen from 1 to 129 and they are all mapped onto a particular symbol. This is the hypothetical reel map:

However, this is only one reel and the majority of slots out there have three reels, each of which is weighted differently. Whenever you move from one reel to the next, the weight gets heavier, which gives you higher chances of getting symbols that pay out higher early on. Keep that in mind.

More Info