Why Some Nonograms Need Guessing

September 15, 2026 · 4 min read

If a nonogram ever forced you to guess, it wasn't necessarily your fault. Using real drafts we drew for Gridora and rejected, we show the two different flaws that push a puzzle into guessing, and how each one is measured.

One complaint, two different problems

"I had to guess somewhere in this puzzle" can point to two different flaws. The first is that the puzzle has more than one solution: the clues don't describe a single picture. The second is sneakier: the puzzle has exactly one solution, but logic alone can't reach it.

Every example below is real. The first three are drafts we drew for Gridora that failed our level check, and every number on this page was recomputed by the solver when the page was built.

Flaw one: more than one solution

We drew a 5×5 star. When we extracted its clues and gave them to the solver, it found three different pictures that fit them:

When a player reaches the top row, no row or column can tell them which square the 1 belongs to. Any choice satisfies the clues, yet the game counts only one as correct. That isn't difficulty; it's a coin toss.

A more striking case: a 5×5 X. Every row and every column has the clue 1 1 or 1. Exactly 48 different solutions fit these clues, and the X we drew is only one of them:

Flaw two: one solution, still guessing

It's tempting to think that one solution means no problem. The 5×5 puzzle below really does have a single solution. But if you go through the rows and columns marking what is certain, at some point you stop:

The only way forward is to assume a square. If you take square 1 of row 1 as filled, a few steps later you contradict some line's clue, so that square must be empty. The result is right, but that's trial and error rather than reasoning: it asks you to open a branch in your head, follow it to the end and back out if needed.

The same happens on bigger boards. The 10×10 crescent moon we drew for Gridora also has one solution and also can't be finished by logic alone, which is why it never shipped.

Why it matters

The joy of a nonogram is knowing why every square is what it is. A puzzle that needs guessing breaks that: when players make a mistake they can't tell whether they reasoned badly or the puzzle is flawed. Both flaws are common in puzzles that are generated at random or published without checking.

In general, deciding whether a nonogram has a solution is a hard computational problem (NP-complete). But checking a single puzzle is easy, so there's no excuse. Every Gridora level has to pass two questions before it ships:

1. Does it have exactly one solution? The solver looks for a second solution; if it finds one, the level is rejected.
2. Can it be finished with row and column logic alone? The solver proceeds without assuming any square; if it gets stuck, the level is rejected.

A puzzle that passes the second question automatically passes the first: a path where every step is forced can't lead anywhere else. We still measure both separately, because knowing which flaw a draft has makes it easier to fix.

How to tell while you play

When you're stuck, suspect yourself before the puzzle: usually there's a column you haven't checked or an empty square you haven't marked. Scan every line once more, especially the overlap of long runs and the ends of completed runs. We cover the core techniques in how to solve nonograms.

If you've scanned every line and can't find a single certain square, you're most likely facing one of the flaws above.