Reels

The Making of a Slot Game: Reels, Weights, and Measures

Slot game reels are composed of different symbols, most often distributed across the reel strip in a fixed order, for example:

Reel stop position 0   A
Reel stop position 1   Fountain
Reel stop position 2   K
Reel stop position 3   Princess
Reel stop position 4   Q
Reel stop position 5   A
Reel stop position 6   Jaguar
Reel stop position 7   J
And so on…

Not all reels on the same game need be the same length, nor do they often have the same weighting of symbols. In fact it's extremely rare to have reel strips that are exactly the same as one another. Weighting (having different numbers of particular symbols on different reels) is necessary to balance game RTP, hit rates, feature hit rates and so forth.

Particular symbol layouts in various sections may be repeated, so sometimes when trying to analyze a game just from the reels it can seem that some stop positions are hitting more than others. In an honestly random system each position on a reel should have exactly that same chance of being spun up as any other.

Typically a reel might be, say, 30-50 symbols in length. There may well be times when a reel strip is much, much longer. For instance, in cases where a symbol on one particular reel is required to trigger it may be that this is far longer than other reels. Having a strip of 200 symbols in length is uncommon, but not unknown.

On a given reel there might only be 1 or 2 substitute (wild) symbols, 1 scatter, and a varying distribution of the smaller symbols. This variance in the distribution of symbols on the reels makes some combination more likely than other, hence why some patterns pay more and some pay less.

In a good, random system the best mechanism that can be used is to randomly determine the stop position of each reel independently and then evaluate winning combination's. So the process would be:

1. A valid bet is placed and the server generates a random stop position for each of the 5 reels
2. There is no variation in the chance of stopping on any given location on each reel
3. Wins are evaluated and awarded as applicable.

It is possible to have reels work in other ways that are still random, but the simplest ways are often the best.

Alternate means include:

Weighted symbol distribution without a fixed order reel strip

In these instances a probability schedule is used to determine what symbols will occur on each position of each reel.

So a game might have a symbol list and probability weighting such as:

Substitute	1%
Scatter	1%
Symbol C	3%
Symbol D	5%
Symbol E	7%
Symbol F	13%
Symbol G	20%
Symbol H	20%
Symbol I	30%

Each of the 3 symbols that will appear on each reel would usually be determined randomly, so 15 different random results might be called.

It’s often an overly complicated solution that doesn’t have too many benefits, and often results in extreme swings in volatility compared to a fixed reel strip order.

Fixed order symbol distribution with weighted stop position probability

In these instances a weighted probability schedule is used to determine where the reel will stop on any given spin. The order of the symbols on the reel is still in a fixed order, but some stop locations are more often used than others.

This is typical of modern versions of the original slots, or “steppers” (named after the computerized mechanism used to stop reels accurately at a given location). They’re still random, but given that the distribution table allows some patterns to appear constantly off winning lines (“near misses”), it’s psychologically unfair to players.