Stacked Symbols
Identical symbols grouped in a vertical block on a reel so they can fill several positions of that reel at once.
Definition
Stacked symbols are copies of the same symbol arranged together on the reel strip, so when that block lands it can occupy two, three, or more positions on a single reel. Full stacks across multiple reels can complete many paylines or ways simultaneously and are a common route to large wins, especially when the stacked symbol is a wild. Stacking is built into how the reel strips are laid out rather than being a separate trigger. Like all slot features, stacking is incorporated into the game's math so the long-run return stays below the amount wagered.
Worked example
A reel spins in a full three-high stack of the same high-value symbol. If neighbouring reels land matching symbols in line, several paylines pay together, and a stacked wild reel can complete lines it does not even share a symbol with.
Why it matters
For a learner, stacks explain the occasional screen full of one symbol and a big hit. For a professional, stack sizing and placement on the reel strip are primary tools for shaping win distribution and volatility.