An empty mark is useful because it rules out placements. It is also dangerous when it has no evidence: one incorrect × can remove the real solution while leaving a different-looking arrangement that seems plausible. Keep undecided cells blank, and use × only when the clues prove a cell empty.
We will use a five-cell line with clue 3 to follow exactly what changes. The complete puzzle used for practice is our 5×5 Arrow.
A blank and an empty mark say different things
A blank means “I have not decided.” It does not restrict the solution. An × means “this cell cannot be filled.” Every later deduction has to respect it.
In this site's nonogram game, choose the empty-mark tool to enter an ×. On a keyboard, focus a board cell and press X; with a mouse, a right-click also marks it empty. Use Undo if you selected the wrong tool or marked the wrong square.
The difference matters even when two boards look almost identical. A row with five undecided cells and clue 3 has three possible placements:
| Placement | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| 1 | ■ | ■ | ■ | × | × |
| 2 | × | ■ | ■ | ■ | × |
| 3 | × | × | ■ | ■ | ■ |
3 legal placements. ■ filled; × empty. These rows are alternatives, not consecutive moves.
Only the middle cell is filled in every possibility. None of the five cells is empty in every possibility. Before considering crossing clues, you can prove one filled cell and no empty cells.
A wrong × can look locally consistent
Suppose you put an × in position 1 because it looks like background. That removes the first placement but leaves two:
| Placement | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| 1 | × | ■ | ■ | ■ | × |
| 2 | × | × | ■ | ■ | ■ |
2 legal placements. ■ filled; × empty. These rows are alternatives, not consecutive moves.
The line still has a solution, so it does not immediately look broken. Positions 3 and 4 now appear forced. But position 4 was not forced before you added the ×. Its apparent certainty depends on the unsupported mark.
This is why “the row still fits” is not enough to validate a guess. An assumption can produce a chain of deductions that are consistent with the assumption but inconsistent with the puzzle. Before trusting a new conclusion, check the evidence for the marks it depends on.
If a crossing column really proves position 1 empty, the very same elimination becomes valid. The appearance of the board is identical; the difference is the reason for the mark.
Other wrong marks create an immediate contradiction
Now consider putting an × in position 3 instead. All three placements require that cell to be filled. The × removes every option: no run of three can fit the five-cell row anymore.
A contradiction tells you that the current assumptions cannot all be true. It does not, by itself, identify the most recent mark as the only possible mistake. An earlier bad mark may have caused several later ones.
On this site, Hint also performs a separate check against the stored answer before offering a deduction. It can point you toward a row containing an incorrect mark. That is an assistance feature, not evidence that the row's visible clues alone prove which mark is wrong. See how our hints work for the distinction.
Do not seal an unfinished run
Another common error is placing empty marks around a run too soon. If a line's only clue is 3 and you have filled two adjacent squares, you have not completed the run. An empty mark on each side would leave no room for the third square.
Multiple clues add a second question: which clue belongs to this run? In a line with clues 2 and 4, a two-cell patch might be the completed first group, or part of the second group. The order and available space have to settle that before you close it off.
Use this check before marking a separator: “Which group is complete, and what prevents it from extending?” If you cannot answer both parts, leave the neighbouring cell undecided.
Recover without erasing the whole puzzle
- Stop adding marks when you notice a contradiction.
- Identify the row or column that can no longer fit its clues.
- Find the first mark in your recent reasoning that was a guess, a picture-based expectation, or an accidental input.
- Use Undo to return to a state before that assumption and its dependent moves.
- Rebuild the next deduction from legal placements rather than from the intended picture.
Undo reverses moves in order; it does not automatically find every mark that depends on a mistake. If you repair one square manually but leave later deductions untouched, those later marks may still be wrong.
Practise proving empty cells
In Arrow, fill the middle row, whose clue is 5. Columns 1 and 5 each have clue 1, now satisfied by that row. Their other cells are proved empty. That is a justified use of ×.
After marking those columns, look at row 2, whose clue is 3. Its two outer squares are excluded, so its middle three must be filled. Compare that with the earlier unsupported ×: here you can trace every exclusion to a completed column clue.
If you need the full sequence, use the five-step Arrow walkthrough. The goal is not to avoid using empty marks. It is to make each one carry a reason you can explain.