If you have played more than a handful of online slots, you have almost certainly played a Book mechanic game — possibly without knowing what to call it. One special symbol acts as both wild and scatter. Land three or more and you get free spins. Before those spins begin, one regular symbol is randomly chosen to expand across its entire reel whenever it appears. That is the formula. It first appeared in Novomatic’s Book of Ra in the mid-2000s, and it has since spawned dozens of direct implementations and hundreds of inspired variants.
What makes it worth understanding properly is that the expanding symbol reveal — that moment before the free spins begin when the game tells you which symbol will expand — is one of the most genuinely tension-filled single events in online gambling. Everything that follows in the free spins round is shaped by what gets chosen in that one instant. Understanding why that moment matters so much requires understanding the full mechanic.
Contents
- Where It Came From
- The Three Components
- The Symbol Reveal: Why It Matters So Much
- How Scatter-Style Pays Work
- Retrigger Systems: Different Approaches
- How Different Providers Implement It
- Comparison Table
- Why the Formula Works Psychologically
- Is It Still Worth Playing?
Where It Came From
Novomatic’s Book of Ra launched in Austrian land-based casinos in the early 2000s before reaching online platforms. The game was built around a concept that was genuinely novel at the time: a single symbol that served both as the wild (substituting for other symbols on paylines) and as the scatter (paying based on count anywhere on the reels, triggering free spins at three or more). This dual-function symbol reduced the paytable complexity while making every appearance of that symbol carry double value.
The expanding symbol in free spins was the second innovation. Rather than just giving players a fixed number of free spins, Book of Ra introduced variance within the bonus round itself: the same trigger could produce a short, low-value round or a long, potentially massive one depending entirely on which symbol was randomly selected to expand. Two players triggering the same free spins on the same spin would have entirely different bonus experiences if different symbols were chosen.
Book of Ra Deluxe refined the original, improved the visual presentation, and achieved widespread casino floor presence across Canada in the late 2000s. The online version followed, and the template was cemented. By 2015, imitators were appearing regularly. By 2020, the mechanic had spread to dozens of providers across multiple themes.
The Three Components
Every Book mechanic implementation has three structural components. They may be dressed differently — an ornate gate in Lord of the Ocean, a crystal ball in Rise of Merlin, the Book of Dead itself in Play’n GO’s version — but the components are consistent:
Component 1: The Dual Wild/Scatter Symbol
One symbol functions as both wild (substitutes for regular symbols on active paylines) and scatter (pays based on count regardless of position; triggers free spins at three or more). The dual function means this symbol has compounding value in the base game: a single appearance on reel three simultaneously extends payline combinations across all lines passing through it and contributes to the scatter count that pays and triggers bonuses. A spin landing three of these symbols simultaneously pays their scatter prize (typically CA$2× stake for 3, CA$20× for 4, CA$200× for 5) and then enters the bonus. The scatter prizes are paid before the free spins begin, so a five-symbol trigger pays CA$200× stake as a standalone prize before the expanding symbol round even starts.
Component 2: The Randomly Selected Expanding Symbol
When free spins trigger, the RNG selects one regular paying symbol to serve as the Special Expanding Symbol for the entire round. This selection happens once per trigger (not per spin) and applies to all spins in that bonus, including any retriggered additions. When the chosen symbol appears on any reel during free spins, it expands vertically to fill all three positions on that reel. The expanded symbol then pays according to scatter rules rather than payline rules — meaning non-adjacent reel coverage counts.
Component 3: Unlimited or Capped Retrigger
Landing three or more of the dual wild/scatter symbol during free spins retriggers the feature. Most implementations award additional spins (typically the same count as the original trigger) and — depending on the specific game — either continue with the same expanding symbol or introduce additional ones. This retrigger potential is where sessions can extend dramatically beyond the initial eight or ten free spins.
The Symbol Reveal: Why It Matters So Much
The expanding symbol reveal is the single most consequential random event in a Book mechanic session. In Lord of the Ocean, the symbols rank from lowest to highest: card royals (10, J, Q, K, A), then Warrior Statue, Mermaid, and Poseidon at the top. A full board of expanded Poseidon symbols pays 5,000× stake. A full board of expanded Kings pays roughly 100× stake. Same trigger, same grid, same free spins count — the only variable is which symbol was chosen, and the output difference is approximately 50-fold.
That is not an exaggeration. Here is what a full-board expansion actually produces at different symbol tiers in a typical implementation at CA$1 stake:
| Symbol chosen | Tier | Approx. 5-of-a-kind value | Full board expansion at CA$1 stake |
|---|---|---|---|
| Card royal (10, J, Q) | Lowest | 5–15× per payline | ~50–150× total (all paylines) |
| Card royal (K, A) | Low | 15–25× per payline | ~150–250× total |
| Treasure Chest / Warrior | Mid | 50–100× per payline | ~500–1,000× total |
| Mermaid / Owl / Dragon | High | 100–200× per payline | ~1,000–2,000× total |
| Poseidon / Merlin / top symbol | Highest | 500× per payline | ~5,000× total |
These are full-board scenarios — all five reels expanded simultaneously, which only happens when the chosen symbol lands on all five reels in the same free spin. Partial expansions (two or three reels) produce proportionally smaller payouts. But the table illustrates why the reveal moment generates the tension it does: before the free spins begin, the game is essentially telling you what the maximum possible outcome of your next several minutes of play could be.
How Scatter-Style Pays Work
This is the mechanic detail that most players do not fully understand, and understanding it explains why the expanding symbol can pay so much. Standard payline wins require symbols on consecutive reels from reel one. A five-of-a-kind combination requires symbols on reels 1, 2, 3, 4, and 5 in sequence. The expanding symbol ignores this requirement after expansion.
When the chosen symbol appears on reels 2, 4, and 5 and expands to fill those reels, the pay is calculated as follows: three instances of the chosen symbol appear on the grid. Under scatter rules, three-of-a-kind pays regardless of whether those reels are adjacent. Poseidon on reels 2, 4, and 5 (non-consecutive) pays three-of-a-kind Poseidon. Under standard payline rules, that combination would pay nothing because the sequence is broken at reel 3.
This scatter-style pay is applied across all active paylines simultaneously. If all three expanded reels produce a three-of-a-kind pay on each of the ten active paylines, the total is that pay value × 10. When expanded symbols reach all five reels, they pay on all ten paylines simultaneously — which is how a single free spin can produce the game’s theoretical maximum in one click.
Retrigger Systems: Different Approaches
The retrigger mechanic is where different Book implementations diverge most significantly. Three main approaches exist:
Three main retrigger systems exist across Book mechanic implementations:
- Same symbol continues (Book of Dead, Lord of the Ocean): Retrigger awards additional spins; the original expanding symbol remains active throughout. No new reveal. If you triggered with Poseidon, Poseidon continues expanding through all retriggered spins. Straightforward and predictable — the symbol chosen at the start determines the ceiling of the entire bonus, including all retriggers.
- New symbol added (Rise of Merlin): Each retrigger introduces a new expanding symbol alongside the existing one. One trigger = one expanding symbol. One retrigger = two simultaneous expanding symbols. The accumulation changes the session ceiling substantially — two high-value symbols expanding simultaneously covers far more of the grid per spin than any single-symbol implementation. Theoretical maximum of nine simultaneous expanding symbols has a probability of approximately 1 in 8.7 million spins.
- New symbol replaces old (some implementations): Retrigger retires the previous expanding symbol and reveals a new one. This introduces per-retrigger variance — upgrading from a card royal to a high-value symbol on retrigger, or vice versa. Less common; not used in the most commercially successful Book implementations.
How Different Providers Implement It
The core mechanic is shared; the execution varies in ways that matter for choosing between games.
Book of Ra Deluxe — Novomatic
The original in its refined form. 10 adjustable paylines, 95.10% RTP, 10 free spins per trigger, same expanding symbol on retrigger. No framing beyond the archaeological Egypt theme. The game that set the template. Still widely available at Novomatic-powered casinos in Canada.
Lord of the Ocean — Novomatic / Greentube
Mechanically identical to Book of Ra Deluxe — same 95.10% RTP, same 10-payline format, same trigger/retrigger system. The difference is the Greek mythology / Poseidon underwater theme. Player reviews frequently describe Lord of the Ocean as “Book of Ra with a different skin,” which is structurally accurate. If you prefer Greek mythology to Egyptian archaeology, this is the direct alternative.
Book of Dead — Play’n GO
Play’n GO’s flagship entry into the format. 10 free spins, same expanding symbol on retrigger, 96.21% RTP at maximum configuration. The slightly higher RTP (compared to the Novomatic originals at 95.10%) and wider casino availability made Book of Dead the dominant commercial version through the late 2010s in Canada. The Egyptian theme continues from its inspiration.
Rise of Merlin — Play’n GO
Play’n GO’s structural contribution to the format: the retrigger-accumulation system. 8 free spins per trigger (fewer than Book of Dead’s 10), 96.58% RTP at maximum configuration, Arthurian mythology theme. The lower initial spin count is offset by the accumulation mechanic — if you retrigger multiple times, the session delivers expanding symbol coverage that no single-symbol implementation can match.
Comparison Table
| Game | Provider | RTP | Paylines | Free Spins | Retrigger Award | Max Win |
|---|---|---|---|---|---|---|
| Book of Ra Deluxe | Novomatic | 95.10% | 10 adj. | 10 | +10 spins, same symbol | ~5,000× |
| Lord of the Ocean | Novomatic / Greentube | 95.10% | 10 adj. | 10 | +10 spins, same symbol | 5,000× |
| Book of Dead | Play’n GO | 96.21% | 10 fixed | 10 | +10 spins, same symbol | ~5,000× |
| Rise of Merlin | Play’n GO | 96.58% | 10 adj. | 8 | +8 spins + new symbol | 5,000× |
Why the Formula Works Psychologically
The Book mechanic generates a specific kind of tension that most slot formats do not replicate. In a standard scatter-triggered free spins round, you know what you are getting before the first free spin — the multiplier, the number of spins, whether wilds are locked. Everything is predetermined at trigger. The expanding symbol reveal adds one moment of genuine suspense between the trigger and the gameplay: for a second or two, you do not know whether you have a profitable free spins round or a potentially exceptional one.
That uncertainty creates anticipation in a format where most players think of free spins as a reward that simply plays out. The reveal converts the free spins from a passive reward into an active moment of revelation. When Poseidon’s face appears as the chosen expanding symbol in Lord of the Ocean, it is a distinct event — not just the start of the bonus, but a moment that changes the stakes of everything that follows.
The scatter-style pay mechanic contributes to this psychology. Because the expanding symbol pays on non-adjacent reels, a player watching the free spins does not need consecutive coverage to win big. Expanded symbols on reels 1, 3, and 5 — visually spread across the grid with gaps — still produce a paying combination. This makes partial expansions feel more rewarding than they would under standard payline rules, and full-grid expansions feel genuinely dramatic because every reel is simultaneously contributing to one large pay event.
Is It Still Worth Playing?
Seventeen years after Book of Ra’s land-based debut, the mechanic remains commercially dominant because it solved several design problems simultaneously. It concentrated session value into an infrequent, variable event (the free spins with expanding symbol). It made that event’s value unpredictable in a way that felt fair (random symbol selection, not player-controlled). It gave the free spins round clear visual drama (the expanding symbol literally fills reels). And it kept the base game functional without complexity — the dual wild/scatter means something meaningful happens on every appearance of that symbol.
For players choosing between implementations, the practical differentiators are RTP (Book of Dead and Rise of Merlin at ~96.2–96.6% outperform the Novomatic originals at 95.10%) and the retrigger mechanic (Rise of Merlin’s accumulation system offers a ceiling not available in single-symbol implementations). The theme is secondary — all four major implementations share the same mathematical ceiling and the same reveal-moment tension. Choose the version at the highest RTP configuration your casino offers, check the active RTP in the game information panel, and the rest is waiting for the right expanding symbol to fill all five reels.