How to play nonograms

A nonogram is a 20×20 grid of cells that’s either filled or empty — together, the filled cells make a small picture. The numbers along the top of each column and the left of each row tell you, in order, how long the runs of filled cells in that line are. 3 on its own means a single run of three filled cells. 2 1 means a run of two, then a gap of at least one empty cell, then a run of one. Your job is to work out which cells are filled.

There are three things you can do to a cell: fill it, mark it with an X (your own bookkeeping for “definitely empty”), or clear it. Pick the tool you want from the toolbar, then click a cell — or press and drag to paint a run of cells in one go. The X marks don’t affect the win condition; they’re just there to help you keep track of cells you’ve ruled out.

The puzzle is solved the moment every cell that should be filled is filled — no need to mark every empty cell with an X. If you’ve over-filled a cell that should be empty, that blocks the win until you clear it. Press Validate on the toolbar at any time to flag any wrong fills in red.

Where to start

The fastest first move on any nonogram is the “edge” deduction: a long run pinned against the corner of a line forces filled cells in the middle of the line even before the exact start position is known. A column with a single clue of 19 on a 20-cell grid, for instance, has a single empty cell at one end — everything else is filled, and the overlap of all valid placements forces every cell except the two endpoints. Look for those long runs first; they crack the puzzle open.

About this puzzle

20×20 medium is a long sitting. The multi-pass dance shows up across many lines simultaneously; you'll often see a cell open up on row 4 that resolves something stuck on column 17. Keep your X marks honest — they're how you keep track of what each line has ruled out.