Marking empty cells
Beginners think a nonogram is about the squares you fill. Experienced solvers know it's just as much about the squares you cross out. A definitely-empty square carries real information — it tells a crossing run where it can't go — and marking those squares with an ✕ is what keeps a puzzle moving. This guide covers when a square must be empty and why it's worth marking every one.
Complete a line, then cross the rest
The simplest empty-marking move: once a line's clues are fully placed, every square that isn't part of a run is empty — mark them all. A clue of just 1 that you've located means the whole rest of that line is crossed out. A clue of 1 1 with both singles placed means every square between and around them is empty.
It sounds obvious, but skipping it is the most common way to stall. Those fresh ✕ marks are exactly what the crossing rows and columns need to make their next move.
Completing a line crosses out the rest
The clue is a single 1, already placed in the center — so every other square is crossed out (✕).
Both runs of 1 are placed at the ends; the three squares between them can't hold anything, so they're all empty.
A gap too small to hold a run
The most useful independent empty-marking deduction: when a stretch of squares is too short to contain the run that would have to go there, it can't be filled — so cross it out. A single empty square can split a line into segments, and any segment shorter than the remaining run is dead space.
In the example below, the line's clue is 3 and one square is already known empty. That empty square isolates the single square before it — one square can't hold a run of three — so it must be empty too. The run of 3 is then confined to the last four squares, where overlap forces two of them.
A gap too small to hold the run
Given the second square is empty (✕): it isolates the first square, which alone can't fit a run of 3 — so it's crossed out too. The run then sits in the last four squares, forcing two.
Empties are clues for the crossing lines
An ✕ isn't just bookkeeping. When you cross out a square in a row, you've also told that square's column that no run can occupy it there. Very often the deduction that unlocks a column isn't a filled square you found — it's an empty one. Treat every ✕ you place as a fact the perpendicular line can now use.
Make it a habit, not an afterthought
Mark empties as you go, not at the end. Leaving them implicit in your head works on a 5×5, but on a 15×15 or 20×20 it's how mistakes and missed deductions creep in. A board where every certain square is either filled or crossed is a board you can actually read — and reading it correctly is most of the battle.