Open almost any online slot library and a version of the same game is sitting somewhere in the list: an adventurer, a dusty tomb, a book that triggers free spins. The theme gets reskinned constantly, but the mechanic underneath it has barely changed since it first appeared two decades ago, and it keeps reappearing under new names because nobody has found a simpler way to get the same payout swing.

Where the Format Started

Novomatic released Book of Ra in 2005 on a 5-reel, 3-row layout with nine adjustable paylines, built around a book symbol that doubles as both wild and scatter — land three or more and ten free spins follow. What made the game worth copying, though, isn’t the trigger; it’s what happens once the bonus round actually starts. The game picks one ordinary symbol at random — the explorer figure, in the original — and turns it into an expanding symbol for that round alone. From then on, wherever it lands, it stretches to cover the whole reel and pays as if every position on it were a winner.

Spin City’s slot collection at https://spin.city/casino/collection/slot shows how far the idea traveled: its shelves carry book-style variants like Book of the Cleo, Book of Hidden Tombs, Book of Mummy, Book of Goddess and Book of Mega Moolah, all built around that same expanding-symbol idea rather than just the original release.

Why One Moving Part Changes So Much

A fixed wild is predictable — it always sits in the same reel, doing the same job every round. A symbol chosen at random for that specific bonus round is different: a player doesn’t know in advance which one it will be, so every free-spin sequence starts with a small piece of genuine uncertainty layered on top of the usual reel outcome. When the chosen symbol lands two or three times across a five-reel screen, each of those turns into a full reel of matching positions, and the payout swings far more than a normal spin would allow. That volatility, sitting on top of an otherwise simple base game, is the part that later releases kept rather than reinvented — it’s cheap to build, easy to explain in a single line of bonus-round text, and it produces the kind of occasional large win that a flat, evenly-paying base game rarely does on its own.

The Clearest Imitator: Book of Dead

Play’n GO released Book of Dead in 2016, eleven years after the original, on a 5-reel layout with ten paylines and an average return of 96.21%. The bonus structure is close to identical in principle: landing three or more book symbols triggers free spins, and before the round starts, the game randomly selects one symbol to expand for its duration, with the round able to retrigger if further book symbols land.

The differences between the two games sit in the surface details:

• The theme — ancient Egypt in both, but with a different cast of characters and art style.
• The paytable and top-symbol value, which shifts how the expanding symbol pays at its best.
• The gamble feature — Novomatic’s original offers a red-or-black card guess that can double a win and be repeated up to five times, while Book of Dead skips it entirely.

Underneath, the mechanism is the same idea applied more than a decade after the original made it work.

What Usually Changes Between Versions

Most book-style slots released since keep the core mechanic fixed and vary a small, predictable set of things instead:

• The theme and symbol set — Egyptian tombs, Aztec temples, pirate maps, and similar settings built around the same explorer-and-book framing.
• The paytable and volatility, since a higher top symbol value or a different expanding-symbol payout changes how the swings feel.
• Reel count and paylines, occasionally, though five reels stayed the standard layout across most releases.
• Extra features layered on top, such as retriggers, multipliers, or a gamble round, added without touching the core expansion mechanic.

That short list covers nearly every book title released across the past two decades. The theme changes, the paytable changes, and the studio credited on the loading screen changes — but the free-spin structure underneath rarely does.

Why the Format Still Holds Up

A mechanic surviving twenty years in an industry that reskins constantly says less about nostalgia and more about design. The expanding symbol gives a simple five-reel game a genuine swing in outcomes without adding new rules a player has to learn mid-round — it’s still just symbols landing on reels, only one of them occasionally fills the whole column. That’s a rare combination: a feature complex enough to create real variance, simple enough that a first-time player understands it after one bonus round. Most genres in this category don’t get copied for twenty years. This one keeps getting copied because the original problem it solved — making a plain five-reel game pay unevenly enough to stay interesting — was never really improved on, just repackaged.