Cluster pays replaces the payline with adjacency. Instead of matching symbols on a fixed line left-to-right, you are paid when a group of the same symbol touches — horizontally or vertically — anywhere on a grid, typically 5×5, 6×6 or 7×7. The minimum cluster is usually five symbols.
Why it feels different
Because winning symbols are removed and replaced (see cascading reels), a single spin can chain into several wins. This makes cluster games feel busy and generous, but the RTP and house edge are set exactly as on any other slot: the maths does not care whether it pays by line or by cluster.
How a cluster is priced
The paytable pays by cluster size in bands — typically 5–6, 7–8, 9–10, 11–12, 13–14 and 15 or more of a symbol — and the top band of the top symbol is the game's ordinary ceiling before multipliers. The grid sets the limit: a 5×5 grid has 25 cells, so a 15+ cluster fills most of the screen; on 7×7 (Reactoonz) or 8×8 (Jammin' Jars) the largest bands are reachable more often, which is why those games can afford the extra features they hang off big clusters — charge meters, symbol removal, and wandering multiplier wilds.
There is no line selection, so there is no "bet per line": the whole stake rides on the grid every spin. A game that quotes its wins as multiples of the total stake is being straightforward; one that quotes "coin" wins against a hidden per-line value is not, and the maximum win is the figure to reconcile them with.
Not the same as "pays anywhere"
Cluster pays needs symbols to touch. A scatter-pays grid — Sweet Bonanza and Gates of Olympus are the well-known ones — pays for the count of a symbol anywhere on the screen with no adjacency at all, usually from eight of a kind on a 6×5 grid. The two are often lumped together because both drop the payline, but they price a screen differently: on a scatter-pays grid a symbol contributes wherever it lands, so a "near miss" of two separate groups of four is not a near miss at all, it is a win. Check which rule a game uses before you read its results as unlucky.
What to check
Read the paytable for the cluster sizes and any multiplier mechanic. High-volatility cluster games can go long stretches with nothing, then resolve a screen-filling chain. That distribution is the design, not a fault.