On every casino floor there is an abundance of slot machines to please a variety of players. A better understanding of common game mechanics will help a player know which machine to choose for their ideal gambling experience. In this article we give a high-level overview of three common game mechanics and how they are affected by the evaluation types covered in our last article, Slots 101.

Stacked Symbols

Stacking symbols on the reel strips has been fundamental in the world of slot games. Generally, there are two types of stacked symbols: static and dynamic. Static stacks are unchanging groups of symbols in the same position on the reel every spin, whereas dynamic stacks are portions of the reel that change into different symbols randomly on every spin. The latter was made popular under Konami’s “Action Stacks” trademark and can be seen in many of their games like China Shores and Dragon’s Law.

On a game that uses “lines,” stacked symbols affect the gameplay in a few ways. The more that symbols of the same type are stacked together, the more volatile the gambling experience will be. This is because having the symbols stacked reduces the hit-rate (the frequency of a non-zero win), since the stacked symbols tend to block smaller wins. The tradeoff is an increase in the potential win. Filling up most or all of the screen with the same stacked symbol is the player’s goal in these games.

On a game that evaluates on “ways,” stacked symbols have a similar yet more dramatic effect. Recall from Slots 101 that when multiple matching symbols land on the same reel, the number of ways for that symbol increases exponentially. On a slot game with five reels each with a height of three symbols (known as a 3×5), a single Lion symbol on every reel would award a single five-of-a-kind Lion pay. Two Lion symbols on every reel awards 32 five-of-a-kind Lion pays. Three Lion symbols on every reel awards 243(!) five-of-a-kind Lion pays.

Wild Multipliers

A multiplier on a wild symbol is perhaps the most widely used slot game mechanic in history. Generally this means that when the wild symbol is involved in a win, that win is multiplied by the respective multiplier. Furthermore, these multipliers commonly combine with each other multiplicatively. For example, in a game with 3X wild symbols, a single wild involved in the win would multiply the total win by three. Two wilds on the same line would multiply the win by nine, and three wilds on the same line would multiply the win by 27.

Wild multipliers also volatilize the math experience, reserving win potential for high paying events involving several wilds. In line games a wild multiplier can mean a massive win potential on a single line. This is quite different from stacked symbols, where the goal is to get a large win on as many lines as possible. Classic three-reel mechanical games use multiplied wilds to achieve a huge pay variety on a very small line count.

For ways games, wild multipliers and wild stacks have a very similar effect. For example, having a stack of three wilds has the same multiplicative effect as having a single 3X wild. In both cases, the total pay is multiplied by three. In the infamous game Buffalo by Aristocrat, all wilds in the Free Games have multipliers. Every Buffalo player hopes for a 3X wild on reels 2, 3 and 4 for the massive 27X multiplier.

Expanding Reels

Expanding reels are when the height of one or more reels changes. This could mean that individual reels grow and shrink randomly or that the entire array doubles or triples in size.

For lines, the effect of expanding reels is not always apparent. The height of the reels does not directly affect the win potential, only the number of lines matters. If the reels double in height at the same time that the lines double in count, then the game will pay out twice as much on average. Spreading out the lines (and not increasing the line count) will reduce volatility with no change in average pay. Watch out for smoke and mirrors and pay attention to the line count.

Expanding reels have a much more dramatic effect on the gameplay of ways games. Remember that the number of ways is the product of the height of each reel. For example, in a 3×5 game, the possible number of unique paths from the left-to-the-right is 3x3x3x3x3 = 243. Let’s say that the height of the third reel is doubled to six. Then the number of ways is 3x3x6x3x3 = 486. Doubling only a single reel’s height doubles the total potential of the game.

With lines, doubling the height of every reel (as well as doubling the line count) doubles the potential of a line game. For our ways game, if we were to double the reel height, we would have 6x6x6x6x6 = 7,776 ways, which is 32 times the potential of the original height! It is because of this relationship between ways evaluations and expanding reels that the combination is so popular in games like Aristocrat’s Buffalo Stampede and Scientific Games’ Dancing Drums.

Although these three mechanics are only a small sample from the Land of Slotz, they are very popular and can be found in most of the games on the floor. Understanding the relationship between mechanics and evaluation type helps bring to light the volatility, win potential and player experience on each slot machine.

Serena Petersen and Anton Kuhlmann are Game Mathematicians. Consulting Game Design and Math at Jackpot Designs, LLC. They can be reached at



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