Expanding Symbols Slots

The mechanic behind the "Book of" games

The expanding symbol mechanic is easy to understand. At the start of a free spins round, the game picks one symbol. Sometimes this happens randomly, and other times you get to choose. Whenever that symbol lands during the bonus, it grows to cover the whole reel. If it lands on a few reels at the same time, they all expand. The game then pays out based on those full columns of matching symbols.

An entire category of slots was built on this one idea. Games like Book of Ra, Book of Dead, Book of Toro and Book of Sam all use basically the same formula. A special book symbol acts as both a wild and a scatter. It triggers the free spins and picks the expanding symbol. The only real differences between these games are the extra features they add, like multipliers, different grid sizes, or the chance to get extra expanding symbols if you retrigger the bonus.

Why the chosen symbol matters

The symbol the game selects changes how the whole bonus round plays out. If you get a high-paying symbol and it expands across multiple reels, you can get very large payouts. If a low-paying symbol is chosen, you will probably see frequent but much smaller wins. When the game picks the symbol randomly, the potential of your bonus round is pretty much decided before you even spin the reels.

Some games let you collect more expanding symbols by retriggering the bonus. Having three or four different symbols expanding on the same screen creates combinations you just do not see in the base game.

You can see how far developers have pushed this idea by looking at games like Banquet of Dead or Book of Arcane 100. They use the exact same core mechanic, but they offer completely different levels of risk and payout potential.