When I started in this industry in 2005, slots were pretty much fixed lines. You spun, the reels stopped and the game checked the paylines. Plenty of games still work that way, but a lot don’t, and the differences aren’t always obvious from the screen.
This guide covers the main slot mechanics, from traditional paylines to cascading reels, multipliers and bonus features. I’ve grouped them by what they do, so you can find the ones you’re interested in and explore how they work in actual games.
If you’re new to the subject, start with Payout Systems. They’re the foundation for understanding how the rest of a slot works.
Each guide explains the mechanic with real games, and most include Face-Off, so you can pick two games and compare how each one handles the feature.
Payout Systems
Before you get into Wilds, bonuses and expanding reels, it helps to understand how a slot recognises a win. Some games use fixed paylines, others count combinations across adjacent reels, while clusters and pay-anywhere systems dispense with those traditional routes altogether.
Paylines are set routes across the reels, and a win only counts if matching symbols sit on one of them, usually starting from the leftmost reel.
Big Bass Bonanza is a typical example: 10 fixed paylines on a 5×3 grid, with wins paying left to right. Fixed means you can’t switch lines off, not that every payline game works the same way.
No lines to follow. Matching symbols just need to land on adjacent reels, usually from the left, and every combination of positions counts.
A classic 5×3 grid gives 243 ways. In Megaways games such as Bonanza Megaways the reel heights change every spin, so the number of ways changes with them.
Matching symbols pay wherever they land, as long as there are enough of them on screen. No lines, no clusters, no adjacent reels. Gates of Olympus and Sweet Bonanza both work this way.
Not the same as a Scatter symbol, which is a single special symbol that usually triggers a feature.
Reel & Grid Dynamics
When I started in the industry in 2005, most online slots were fairly straightforward. The reels stopped, the game checked for wins, and that was that. Cascading Reels were one of the changes that made things more interesting, allowing a single spin to set off a whole sequence of reactions. The other mechanics here also change what happens on the screen, from expanding reels to grids that grow after a win.
Winning symbols disappear, new ones drop into the gaps and the game checks again, all from the same spin.
Pragmatic Play calls it Tumble (Gates of Olympus), NetEnt calls it Avalanche (Gonzo’s Quest). It can sit on top of paylines, ways, clusters or pay-anywhere wins.
The grid itself gets bigger: extra rows, a taller reel or cells that unlock, sometimes for a whole feature and sometimes for a single sequence.
Dragon’s Fire InfiniReels starts on a 3×3 grid and goes back to it when the sequence ends. Not to be confused with Expanding Symbols, where the symbol grows and the grid stays put.
A narrower version of Expanding Reels: the win itself makes the grid grow, rather than a feature or a collection.
AvatarUX’s PopWins games, such as HelioPOPolis, are the best-known example, but other studios use the same idea under their own names.
Symbol Behaviours
Sometimes the grid behaves normally and the symbols do the interesting work. A Wild can stay in place, a symbol can spread across a reel, another can arrive oversized, and a Mystery Symbol can hide what it is until it lands. Once the animation finishes these can look alike, but they are doing different jobs.
A Wild that stays where it lands for more than one spin.
In The Dog House Free Spins, Wilds on reels 2 to 4 stay put, multipliers and all, for the rest of the feature. In other games it might be one respin. Sticky describes how long it stays, not what else it does.
One symbol that covers several positions at once, such as 2×2 or 3×3.
In Wolf Gold’s Free Spins, the middle three reels spin together as one giant 3×3 symbol. Reactoonz builds its Giantoonz from four matching symbols in a square. Giant, Colossal, Big and Mega are names, not standard sizes.
Symbols that land hidden and then reveal what they are, which might be a paying symbol, a Wild or something special.
Razor Shark is built around them: its Mystery Stacks move down the reels during the Free Games until none are left.
Multipliers
Multipliers seem to be everywhere these days, sometimes to the point of overkill. But there’s more to them than sticking an x2 or x5 on the screen. Gates of Olympus draws multiplier values at random and adds them together, while Bonanza builds its multiplier through successive winning reactions during Free Spins.
A number such as x2 or x5 applied to a win, a prize or another value in the game.
The interesting part is how they combine. In The Dog House, Wilds worth x2 and x3 on the same line make x5. In Buffalo King’s Free Spins, the same two make x6.
A multiplier that climbs as something happens in the game.
In Bonanza’s Free Spins it goes up by one with every winning reaction, with no published cap. Other games raise it on cascades, on collected symbols or as a feature progresses.
The value is drawn from a set rather than built up.
Gates of Olympus multiplier symbols can carry anything from x2 to x500, added together once the tumbling stops. What a random multiplier applies to, and how long it lasts, are separate rules.
Collection & Respins
I’ve always had a soft spot for collection features. They remind me of the pub fruit machines I played as a student, where nudges and feature trails gave you something to work towards. I sometimes wish more of that character had carried over into online slots.
Modern collection mechanics work differently, of course. Big Bass Bonanza is a useful example because it combines Symbol Collection and Cash Collect in the same Free Spins feature.
The game keeps count of something and pays off when the count reaches a target.
Big Bass Bonanza counts Fishermen during Free Spins: the 4th, 8th and 12th each add 10 spins and raise the collection multiplier to x2, x3 and finally x10.
Symbols carry visible money values, and a separate collector picks them up.
Big Bass Bonanza again: Fish symbols show cash values and the Fisherman Wild collects them. One symbol carries the value and another collects it, which is what sets this apart from Symbol Collection.
Prize symbols lock in place while the rest of the screen respins. Also known as Money Respins or Hold and Spin.
In Wolf Gold, six or more Money symbols start the round with three respins, and every new one resets the count to three. Fill all 15 positions and you get the Mega Jackpot.
Bonus & Feature Access
“Bonus feature” covers a lot of ground. A game might award Free Spins, ask you to choose between two versions of a bonus, charge more for a spin with altered conditions or, outside Great Britain, sell you direct entry into a feature.
A run of spins you don’t pay for individually, and a feature that can work very differently from one game to another.
The Dog House picks a total of 9 to 27 before the feature starts and never adds more. Chaos Crew starts you on three and resets the count every time you win.
You choose which version of a feature you get: more spins or bigger modifiers, one bonus path or another.
A genuine Bonus Selection lets you choose between known feature options. Picking a concealed option that merely reveals the mode, as in Thor’s Rage 2, is different.
Paying a fixed price to go straight into a bonus. Providers call it Bonus Buy, Buy Feature or Buy Free Spins.
It’s not available in games offered by GB-licensed operators. Big Bass Splash shows why international versions differ: its general material lists Buy Free Spins, but its UK certification says the feature shouldn’t be enabled.
Slot mechanics often work together
The six groups above are useful for finding your way around. Actual games are not nearly so considerate.
The trick is to stop asking for one label that explains the whole game. Instead, break it into jobs: how is the win recognised, what happens to the symbols afterwards, does the grid change, and is anything carried into the next step?
It’s also why a new theme doesn’t always mean a new game underneath. Once you understand how the wins, symbols and features behave, familiar mechanics become much easier to spot.
How I research
Where the guides come from
Every guide starts from the games’ own rules and developer material, then my own checks in the games themselves.
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