Ask any longtime player to name a mechanic that quietly defined a genre, and the examples arrive fast. The grappling traversal of a Metroidvania. The permadeath loop that made roguelikes a category rather than a curiosity.
The waist-high cover that reshaped third-person shooters after one hit release. A single clever idea, copied and refined across hundreds of titles, becomes a genre of its own.
Slot machines have the same story, and one of the clearest cases is Book of Ra. Shuffle catalogues its Book of Ra online page under its Novomatic listings, which makes it easy to study the original template before comparing it to the many games it inspired.
If you spend time reading about how mechanics travel between titles, the way readers do in the game coverage here, the slot version of that migration is worth understanding on its own terms.
The slot itself was built by the Austrian developer Novomatic, whose corporate history and gaming-cabinet roots are summarised on the company’s Wikipedia page. Its Egyptian-themed reels introduced an idea that rival studios could not stop borrowing.
Where one mechanic becomes a whole category?
Genres rarely start with a manifesto. They start with a feeling that a design does something others do not, and then a wave of imitators sanding down the rough edges. In video games the pattern is obvious in hindsight.
In slots it works the same way, except the defining trick tends to be a single bonus behaviour rather than a control scheme or a level structure.
Book of Ra’s contribution was the expanding symbol. During its free-spins round, one symbol is chosen at random, and whenever it lands it stretches to fill an entire reel top to bottom. That one behaviour changed how a bonus round feels, and it turned out to be endlessly reusable.
What the expanding symbol actually does?
On a standard payline, a symbol pays only where it physically sits. The expanding version breaks that rule for the length of the feature.
When the special symbol appears, it covers every position on its reel, so it can complete lines it never touched directly and pay across non-adjacent rows.
The result is a bonus round with a very different rhythm from the base game. Most spins during the feature do little, then one lucky expansion can chain several lines at once.
Designers understood something players feel instinctively: a rare, dramatic payout reads as more exciting than a steady drip, even when the underlying return is unchanged.
From cabinet halls to online reels
Novomatic did not begin online. Its early business was physical gaming cabinets in arcades and gaming halls, the same tactile hardware culture that emulator and retro-console fans still care about.
When the catalog moved to browsers, the expanding symbol traveled cleanly because it needed no new hardware, only code. An RNG, or random number generator, picks the special symbol, and the reel animation does the rest.
That portability is exactly why the mechanic spread. A studio wanting its own Egyptian adventure slot could adopt the expanding-symbol idea, change the art, adjust the numbers, and ship.
Over time, the “book” style slot became a recognizable sub-genre with shared DNA, much like how a wave of soulslikes followed one influential template.
The math behind the drama
Here is the part worth being honest about. A dramatic bonus feature does not change who wins over time.
Book of Ra runs at a return to player, or RTP, in the region of 95 to 96 percent, which means the house edge is built into every session regardless of how a single free-spins round goes. RTP is a long-run average across millions of spins, not a promise for your afternoon.
No pattern, no timing, and no “system” removes that edge, because every outcome is RNG-driven and independent of the last.
Understanding the expanding symbol makes you a more informed player and a better reader of game design. It does not tilt the odds. The interesting skill here is appreciating the craft, not beating the arithmetic, which cannot be beaten.
Expanding symbols versus standard wilds
Newcomers often lump the expanding symbol together with a wild, since both boost payouts. They behave differently, and the contrast explains why the expanding version felt fresh enough to launch imitators.
| Standard wild | Expanding special symbol |
|---|---|
| Substitutes on the single position it lands | Stretches to cover its entire reel |
| Works in base game and bonus alike | Usually tied to the free-spins feature |
| Pays where it physically appears | Can complete lines it never directly touches |
| Steady, incremental contribution | Rare, high-variance bursts of payout |
| Common across most modern slots | The signature move of the “book” sub-genre |
Why designers keep remixing it?
Good mechanics survive because they are simple to grasp and hard to exhaust. The expanding symbol scales with theme, from pharaohs to explorers to folklore, and it gives a bonus round a built-in climax.
That is the same reason genre-defining video game ideas keep reappearing with new skins: the core loop still satisfies.
For players, the takeaway is less about chasing the next clone and more about recognising the pattern. Once you see the expanding symbol as the ancestor of a whole shelf of lookalikes, the sub-genre reads as a family tree rather than a pile of near-identical titles.
Gambling involves risk. 18+. Play responsibly, treat any spend as entertainment, and set limits before you start.
FAQs
What is the expanding symbol in Book of Ra?
It is a bonus behaviour in the free-spins round. One randomly chosen symbol expands to fill its whole reel when it lands, letting it pay across lines it would not normally complete.
Does the expanding symbol improve my odds of winning?
No. Outcomes are RNG-driven and the house edge is fixed. The feature changes how the bonus feels, not the long-run math, which is roughly 95 to 96 percent RTP.
Why is Book of Ra called genre-defining?
Its expanding-symbol mechanic was widely copied, creating a recognisable “book” style of adventure slot with shared design DNA, similar to how one title can spawn a video game sub-genre.
Who made Book of Ra?
It was developed by Novomatic, an Austrian company with roots in physical gaming cabinets before its titles moved online.
Is an expanding symbol the same as a wild?
Not quite. A wild substitutes on the spot it lands, while an expanding symbol stretches to cover an entire reel, usually only during the free-spins feature.
