Advanced nonogram techniques
Got the basics down? Here are the four techniques that crack 15×15 and 20×20 grids — each demonstrated on an interactive mini-grid.
Overlap, the general version
For a block of size B in a free space of size L: slide it all the way left, then all the way right. The intersection of both positions — 2 × B − L cells whenever that’s positive — is guaranteed. This is THE big-grid technique: apply it to every large block right away.
Edge logic
As soon as a filled cell touches a border (or a ✕ acting as one), the line’s first block is anchored: its length unrolls, and a ✕ closes it. Hunt for these anchors after every sweep — they appear constantly.
Forbidden cells
A cell is forbidden when no valid placement covers it: too far from any block, squeezed between two ✕ in a gap too small, or adjacent to an already-complete block. Every ✕ you place tightens the columns crossing that line.
Block splitting
✕s cut a line into independent sections. Mentally distribute the remaining blocks among those sections — often only one assignment works, and each section becomes a small line where overlap applies again:
The method when “nothing works”
- Revisit lines whose blocks are all placed — close them with ✕s.
- Re-sweep every column touched by your latest additions.
- Recompute overlaps: they improve as free space shrinks.
- Still stuck? Paste the grid into the solver in step-by-step mode: it shows the next deduction and the technique producing it — without spoiling the rest.
Frequently asked questions
What’s the overlap formula?
For a block of size B in a space of size L: if 2 × B > L, then 2 × B − L center cells are guaranteed. Example: a block of 6 in 10 cells → 2 safe center cells. If the block is half the space or less, overlap yields nothing.
How can I tell whether a grid has several solutions?
If pure logic brings you to a point where two different hypotheses each lead to a consistent grid, the clues don’t pin down a unique picture. Our solver detects this automatically — paste your clues and it delivers the verdict.
Why do I always get stuck in the middle of the grid?
Almost always because ✕s are missing: without them, the remaining space looks freer than it really is. Revisit every line whose blocks are complete and close it out, then re-sweep the crossing lines.
Should rows be processed before columns?
No — order doesn’t matter, alternation does. Every cell found in a row is fresh information for its column, and vice-versa. The winning habit: after changing any line, visit the crossing lines it touched.
Practice
The best training is grids at your level. Medium 15×15 to build the reflexes, hard 20×20 when you fancy suffering a little, the Daily challenge for consistency — or a printable sheet to play with a pencil. The basics live in the beginner guide.