The unique solution
A well-made nonogram has exactly one answer, and that single fact is what makes the puzzle fair. It's the reason every square can be worked out by logic, and the reason guessing is never required. This guide explains what a unique solution really means, shows what happens when a puzzle doesn't have one, and how we make sure ours always do.
One picture, and only one
A nonogram's clues describe a picture, but not every set of clues describes exactly one picture. "Unique solution" means there is precisely one way to fill the grid so that every row and column clue is satisfied at the same time — no more, no less. That's the property a proper puzzle must have.
When it holds, solving is pure deduction: at every step there's a square whose state is forced, and following those forced squares leads to the one and only answer.
What ambiguity looks like
The smallest puzzle that breaks the rule is a 2×2 grid where every row and every column has the clue 1. That's satisfied by two completely different pictures — the two diagonals below. Both are perfectly valid answers to the same clues, so there's no way to know which one is "correct."
That's an ambiguous puzzle, and it's unfair: a solver following flawless logic could arrive at either picture. A puzzle like this can't be solved without guessing — which is exactly why puzzles with more than one solution shouldn't be published.
This is why you never have to guess
Uniqueness and no-guessing are two sides of the same coin. If a puzzle has one solution and you reach a point where you'd have to guess, it means a deduction exists that you haven't spotted yet — not that the puzzle is stuck. The forced square is always somewhere on the board. So when you feel tempted to guess on a well-made puzzle, the productive move is to keep looking for the line that resolves without a gamble.
How every puzzle here is checked
We don't take uniqueness on faith. Before a puzzle is published, a solver runs over it and confirms there is exactly one solution — anything with zero solutions (contradictory clues) or more than one (ambiguous) is rejected outright. It's the single most important gate in how we build puzzles, described in more detail on the "how we make our nonograms" page. The upshot for you: on this site, if it feels like you need to guess, you don't — the answer is always reachable by logic.