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Best of John Robison

Gaming Guru

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Slot Machine Payline

27 August 1999

Are all the symbols on a slot reel equally likely to land on the payline?

Probably not.

Before slots were computerized, slot manufacturers were looking for ways to be able to increase the jackpots offered on their machines. In order to pay a larger jackpot, they had to make the jackpot harder to hit. There were only two ways they could do that. First, they could add another reel to the machine. Second, they could make the reel larger so it would have more stops.

Neither the players nor the manufacturers liked these solutions. Players instinctively knew that adding an additional reel to a machine or adding symbols to the reel made the jackpot harder to hit. The manufacturers didn't like the size these new games would have to be to accommodate the extra reel or larger reels. IGT was working on a behemoth slot machine when the microprocessor came to their rescue.

Mr. Inge Telnaes received patent 4,448,419 on May 15, 1984. The title of the patent is Electronic Gaming Device Utilizing A Random Number Generator For Selecting The Reel Stop Positions. Telnaes patented a method to make a slot machine reel appear to have more stops on it than it really has by using a computer program.

Here is an excerpt from the patent:

"It should be noted that the market demands higher and higher payoffs to maintain and increase player appeal, yet the casino or operator must be assured that the probability of win and payout allows for a reasonable business profit....

It is therefore the purpose of this invention to increase the capability of the designer to include high payoffs without increased physical size of the machine....It should be noted that the players perceive larger machines as being less "good" in terms of winning and payout chances. That is, large physical machines and a large number of reels develop an attitude in the player which affects the play and acceptance of the machine although this does not always coincide with the true mathematical reality and probability of payout of the machine....this attitude may be more influential on whether or not the machine is played than published figures showing the payoff odds. Thus, it is important to make a machine that is perceived to present greater chances of payoff than it actually has within the legal limitations that games of chance must operate."

Here's how slot manufacturers use Telnaes technology in their slot machines: The game designers create a virtual slot reel in the computer program. They take each physical stop on the slot reel and assign it to one or more virtual stops on the virtual reel. They might put the jackpot symbol on the virtual reel twice and put the blanks above and below the jackpot symbol on the virtual reel five times each.

Here's an example from a real slot machine. First, the physical reel layout. It's called a Physical Reel Strip Listing because the symbols are on a strip of paper that is attached to the reels.

PHYSICAL REEL STRIP LISTING REEL STRIP # :6259 --------------------------------------------------- Line # 1 ~~ ~~ ~~ Line # 2 3B 3B 3B Line # 3 ~~ ~~ ~~ Line # 4 7 7 7 Line # 5 ~~ ~~ ~~ Line # 6 2B 2B 2B Line # 7 ~~ ~~ ~~ Line # 8 1B 1B 1B Line # 9 ~~ ~~ ~~ Line # 10 TJ TJ TJ Line # 11 ~~ ~~ ~~ Line # 12 CH CH CH Line # 13 ~~ ~~ ~~ Line # 14 1B 1B 1B Line # 15 ~~ ~~ ~~ Line # 16 7 7 7 Line # 17 ~~ ~~ ~~ Line # 18 2B 2B 2B Line # 19 ~~ ~~ ~~ Line # 20 TJ TJ TJ Line # 21 ~~ ~~ ~~ Line # 22 1B 1B 1B

There are 22 physical stops on this reel. Here's how the game designers take those 22 stops and turn them into 72 virtual stops.

EXPANDED REEL STRIP LISTING REEL STRIP # :6259 -------------------------------------------------- Line # 1 ~~ ~~ ~~ Line # 49 7 ~~ ~~ Line # 2 ~~ ~~ ~~ Line # 50 ~~ 7 7 Line # 3 3B ~~ 3B Line # 51 ~~ ~~ ~~ Line # 4 3B 3B 3B Line # 52 ~~ ~~ ~~ Line # 5 3B 3B 3B Line # 53 ~~ ~~ ~~ Line # 6 ~~ 3B ~~ Line # 54 2B ~~ ~~ Line # 7 ~~ 3B ~~ Line # 55 2B 2B ~~ Line # 8 ~~ 3B ~~ Line # 56 2B 2B 2B Line # 9 7 3B ~~ Line # 57 ~~ 2B 2B Line # 10 ~~ 3B ~~ Line # 58 ~~ 2B 2B Line # 11 ~~ 3B 7 Line # 59 ~~ 2B ~~ Line # 12 ~~ ~~ ~~ Line # 60 ~~ ~~ ~~ Line # 13 ~~ ~~ ~~ Line # 61 ~~ ~~ ~~ Line # 14 2B ~~ ~~ Line # 62 TJ ~~ ~~ Line # 15 2B 7 ~~ Line # 63 ~~ ~~ ~~ Line # 16 2B ~~ ~~ Line # 64 ~~ ~~ TJ Line # 17 2B ~~ 2B Line # 65 ~~ TJ ~~ Line # 18 ~~ ~~ 2B Line # 66 ~~ ~~ ~~ Line # 19 ~~ ~~ 2B Line # 67 ~~ ~~ ~~ Line # 20 1B ~~ ~~ Line # 68 1B ~~ ~~ Line # 21 1B 2B 1B Line # 69 1B ~~ ~~ Line # 22 1B 2B 1B Line # 70 1B ~~ 1B Line # 23 1B 2B 1B Line # 71 1B 1B 1B Line # 24 ~~ 2B 1B Line # 72 1B 1B 1B Line # 25 ~~ 2B ~~ Line # 26 ~~ 2B ~~ Line # 27 ~~ ~~ ~~ Line # 28 ~~ ~~ ~~ Line # 29 TJ 1B ~~ Line # 30 ~~ ~~ TJ Line # 31 ~~ ~~ ~~ Line # 32 ~~ ~~ ~~ Line # 33 ~~ ~~ ~~ Line # 34 ~~ ~~ ~~ Line # 35 CH TJ ~~ Line # 36 CH ~~ CH Line # 37 CH ~~ CH Line # 38 CH ~~ CH Line # 39 ~~ ~~ ~~ Line # 40 ~~ ~~ ~~ Line # 41 ~~ CH 1B Line # 42 1B CH 1B Line # 43 1B CH 1B Line # 44 1B ~~ 1B Line # 45 1B ~~ ~~ Line # 46 ~~ 1B ~~ Line # 47 ~~ ~~ ~~ Line # 48 ~~ ~~ ~~

Some of the symbols appear more than once on the virtual reel. The symbols appear on the same order in the virtual reel as on the physical reel, but some symbols are repeated in clumps. It's almost as if someone stuttered while making the list.

Let's look at how the virtual reel makes the probability of the of a symbol landing on the payline less than what it appears to be. It appears as if the probability of landing a Triple Jackpot symbol (TJ) on the payline on reel 1 is 2/22. There are two TJs on the reel and there are 22 stops on the reel. But the virtual reel has 72 stops and only two TJs, so the real probability is 2/72.

The Random Number Generator (RNG) in the slot machine's control program chooses a random number. The number is used to select a virtual stop--not a physical stop--on each reel. Say the random number determines that we want virtual stop 37 on reel 1. Looking at the Expanded Reel Listing, we see that it will be a cherry (CH). There's a table in the control program that says virtual stops 35, 36, 37, and 38 correspond to physical stop 12. The control program then stops reel 1 at physical stop 12. The same process is repeated to get the symbols for the other reels.

By using the output from the RNG to choose a virtual stop and then mapping that virtual stop back to a physical stop, the game designers are able to alter the probabilities of the symbols landing on the payline from what the probabilities appear to be based on the physical reel.

In the beginning of this answer, I said that the symbols were probably not equally likely to land on the payline. Sometimes they are. The Telnaes patent is used to map virtual reels to physical reels. On a video slot machine, there is no physical reel. Video slots can have an unlimited size reel and, thus, have no need to map from a virtual reel to a physical reel.

The symbols may also be equally likely on certain machines with bonus games, like Jackpot Party. IGT owns the Telnaes patent and other manufacturers have to pay a license fee to IGT in order to use a virtual reel mapped to a physical reel in their slot machine. According to Randy Adams, game design guru at Anchor Gaming, you need to use Telnaes technology in order to have a big jackpot on the reel-spinning game. But if you move the big jackpot to the bonus game, you can use a one-to-one mapping on the reels. And you don't need Telnaes technology to assign bonus values to treasure chests or oyster shells.


Send your slot and video poker questions to me at slotexpert@home.com or in care of rgt@RGTonline.com.
John Robison

John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots
John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots