Slot machines started as a pretty simple concept. You put your money in, pulled the handle and if you lined a winning combination up you were paid. The concept is still the same but there's more than meets the eye. I won't bore you with the history of how it all started so we'll fast forward to modern reel slot machines.
There's more flavors of modern slots than you can count now. Wilds, triples, doubles, five times, ten times, twelve times, hundred times, triple double, triple triple, the list goes on and on. Then you get into pay lines, 1 pay line, 3 pay lines, 5 pay lines, 9 pay lines, 20 pay lines, bonus reels, 4 reels, 5 reels, 18 reels... it could quickly get confusing. But all slot machines have one thing in common, the random number generator (RNG). Every gambling machine has one whether it's a reel slot, video slot, video poker, video blackjack, video craps.. whatever, it has an RNG buried deep down inside of it.
Here's how the RNG works. For this example we'll use the Double Dollars Red White & Blue machine from IGT.
The original non-computerized version of slot machines were only able to have as many stop as was on the physical reel strips. Almost all reel strips on 3 reel games have 22 stops, with 1 jackpot symbol on each reel strip, so the odds of hitting the top jackpot was 22X22X22=10,648 or every 10,648 spins the top pay should stop on the payline. The odds were pretty short in those days and the jackpots were pretty low, so IGT came up with the theory of virtual reel strips and the RNG. All the RNG does it generate numbers randomly and takes those numbers and applies them to virtual reel strips that'll map back to the original 22 stops on the reel strips.
Modern slot machines can have up to 512 stops on the reels. So with 1 jackpot symbol or stop per reel the odds of hitting the 3 jackpot stop combination would be 512X512X512=134,217,728 or every 134,217,728 spins the top jackpot would hit.
Luckily they only use that many reel stops for games like Megabucks, Wheel of Fortune and games with huge wide area progressive jackpots. The most common number of stops on the virtual reel strips are 72 stops for most games on the casino floor. The games with huge multipliers like Ten Times Pay uses many more stops because of the high multiplier.
Here's how it goes down.
You insert your money, the RNG is humming away and it never stops creating game outcomes. You place your bet, and press spin or pull the handle. It's at that nanosecond that the RNG selects your game outcome. It spits out 3 numbers and sends it to the brain of the slot. Lets say for example it picks 64 50 04. One number for each of the 3 reels. On five reel games it would select 2 more numbers for the extra reels, but 5 reels have a larger number of stops.
Anyway, the brains of the computer looks at it's map of the virtual stops and actual stops, which looks like this:
Line # 1 ~~ ~~ ~~ Line # 40 ~~ B7 ~~ Line # 2 ~~ ~~ ~~ Line # 41 B7 B7 ~~ Line # 3 B7 B7 ~~ Line # 42 B7 B7 ~~ Line # 4 B7 B7 B7 Line # 43 B7 ~~ DS Line # 5 B7 B7 ~~ Line # 44 B7 RB ~~ Line # 6 ~~ ~~ ~~ Line # 45 ~~ RB ~~ Line # 7 ~~ RB ~~ Line # 46 ~~ RB ~~ Line # 8 ~~ RB ~~ Line # 47 ~~ RB ~~ Line # 9 RB RB RB Line # 48 DS RB ~~ Line # 10 RB RB RB Line # 49 ~~ RB RB Line # 11 RB RB RB Line # 50 ~~ ~~ RB Line # 12 ~~ RB RB Line # 51 ~~ ~~ RB Line # 13 ~~ ~~ RB Line # 52 ~~ ~~ RB Line # 14 ~~ ~~ RB Line # 53 BB ~~ RB Line # 15 H7 ~~ ~~ Line # 54 BB ~~ RB Line # 16 H7 ~~ ~~ Line # 55 BB DS RB Line # 17 ~~ H7 ~~ Line # 56 BB ~~ ~~ Line # 18 ~~ ~~ H7 Line # 57 BB ~~ ~~ Line # 19 BB ~~ ~~ Line # 58 BB ~~ ~~ Line # 20 BB ~~ ~~ Line # 59 BB ~~ H7 Line # 21 BB BB BB Line # 60 ~~ ~~ H7 Line # 22 BB BB BB Line # 61 H7 R7 ~~ Line # 23 BB BB ~~ Line # 62 H7 R7 ~~ Line # 24 BB BB ~~ Line # 63 H7 ~~ ~~ Line # 25 BB BB ~~ Line # 64 ~~ ~~ BB Line # 26 ~~ ~~ R7 Line # 65 ~~ ~~ ~~ Line # 27 ~~ ~~ R7 Line # 66 HB BB ~~ Line # 28 ~~ R7 R7 Line # 67 HB BB HB Line # 29 R7 R7 ~~ Line # 68 HB BB HB Line # 30 ~~ ~~ ~~ Line # 69 HB BB HB Line # 31 ~~ ~~ ~~ Line # 70 HB BB HB Line # 32 ~~ ~~ HB Line # 71 HB BB HB Line # 33 HB HB HB Line # 72 HB BB HB Line # 34 HB HB HB Line # 35 HB HB HB ~ = BLANK RB = RED BAR Line # 36 HB ~~ HB HB = WHITE BAR BB = BLUE BAR Line # 37 HB ~~ HB R7 = RED 7 H7 = WHITE 7 Line # 38 HB ~~ ~~ B7 = BLUE 7 DS = DOUBLE DOLLARS Line # 39 HB B7 ~~
The virtual stops the RNG called up when you started your game are listed above. So 64 50 04 would map back to BLANK BLANK BLUE 7 the computer would see that and send it to the reel motors to stop at the appropriate stops on the physical reel strips as seen below.
If you take this strip and fold it over that's how you make a reel.
Which would come back to stops seen above. The blanks are pretty general, stop #64 & #50 has many physical places it can stop but most times it would pick physical stop 3 or 5 to give the effect of a near miss. #04 would only have 1 place it could stop on the physical reel and that would be stop number 12 since that's the only BLUE 7 on the physical reel strip. And in about 6 seconds of window dressing for the player of the reels spinning the outcome would be displayed on the machine.
It's worth noting that the higher pays, particularly the combination of RED 7 WHITE 7 BLUE 7, as seen on the machine above, only have 1 stop per reel on the physical reels and 1 stop per reel on the virtual reels. So to attain the RWB 7 pay the RNG would have to select virtual stop #28, #17 and #04 and the physical reels would spin to stops #20, #16 and #12. So the odds of getting this combination using the virtual strips above would be 72X72X72=373,248 or once every 373,248 plays.
Incidentally, the odds are exactly the same for hitting any combination of 4 times the red white and blue 7's and the top jackpot. So if you do happen to hit the jackpot on this game, thank your lucky stars that the B7 didn't decide to stop with the 2 doubles and just pay you 3,200 coins instead of 5,000 for the 3 doubles.
Using this particular set of virtual reel stops, there are 373,248 possible combinations. This set of virtual reels came from a 97.157% return game. Each game has 5 or 6 different returns, game makers manipulate the returns by adding and subtracting the smaller pays, like anybar and single bar hits from the virtual strips. The top jackpot odds almost always remain the same no matter what % the machine is set to return.
And that's pretty much how the slot machine works on the inside. It's a pretty complicated process that takes place in less than a second. In the time it took you to read this page about 3 million game outcomes have been generated by 1 machine.