Nonogram

How to play Nonogram

Nonograms — also known as Picross, Griddlers and Hanjie — are picture logic puzzles. Numbers along the top and side tell you how the filled squares in each line are grouped, and working out which squares those are reveals a small picture.

Every puzzle here can be solved by deduction alone. You should never have to guess, and if you find yourself guessing there is a deduction you have not spotted yet.

Reading the clues

The numbers beside a row are the lengths of the runs of filled squares in that row, in order, with at least one blank between each run. A clue of 4 2 on a ten-square row means four filled squares, then a gap of one or more, then two filled squares — with the whole group free to sit anywhere in the row as long as that order holds.

A clue of 0 means the line is empty. The same reading applies to columns, top to bottom.

Where to start

The opening moves in a nonogram are almost mechanical, and they are the same every time.

• Look for full lines. A clue equal to the line length fills the whole line. On a 10-wide row, a clue of 10 is free.
• Overlap the big clues. Push a run as far left as it will go, then as far right, and any square filled in both cases must be filled. A clue of 8 on a 10-wide row gives you six certain squares immediately.
• Mark the blanks. A square you have proved empty is as useful as one you have proved filled, and marking it stops you reconsidering. The cross tool is for exactly this.
• Alternate rows and columns. A nonogram is solved by bouncing between the two. What a row gives you makes a column resolvable, and back again.

Sizes and difficulty

Grid size and difficulty are separate choices. Size sets how long the puzzle is; difficulty sets how hard the deductions get.

• 5 x 5 — a quick one, and the right place to learn the rules.
• 10 x 10 — the standard picture-logic size.
• 15 x 15 — a proper sit-down puzzle.
• Easy falls out with straightforward deductions, Medium needs a few rounds of back-and-forth, and Hard needs many alternating row and column deductions before it opens up.

Solvable by logic, and why that is a promise

Difficulty here is measured rather than guessed at. When a puzzle is generated, a solver tries to finish it using nothing but constraint propagation — looking at one line at a time and filling in only what is forced, never trying a square to see what happens.

A candidate puzzle is kept only if that solver drives it all the way to a complete grid. Since every square it filled was forced, no second solution can exist, so a unique answer comes free with the guarantee.

Difficulty is then how many full passes the solver needed: a puzzle that falls out in two passes is easy, one needing many rounds of alternating row and column work is hard. The bands scale with grid size, because a 15 x 15 naturally takes more passes than a 5 x 5.

The daily

One shared puzzle a day, the same for every player, with a 10 x 10 on weekdays and a bigger 15 x 15 at weekends, ramping through the week. Finishing it extends your streak.

The daily is saved separately from free play, so starting a practice puzzle never costs you a half-finished daily.

Controls

• Fill and Cross switch what a tap does. Crosses are your own notes and are never checked.
• Drag across the grid to fill or cross several squares in one go — the value is decided by the first square you touch.
• Check tells you how many filled squares are wrong, without telling you which.

Common questions

Do I ever have to guess?

No. Every puzzle is verified at generation time to be solvable by pure deduction, which also means it has exactly one solution.

What are the crosses for?

Marking squares you have proved empty. They are an aid to your own reasoning — the puzzle is judged on the filled squares alone.

What is the picture meant to be?

Usually nothing in particular. The grids are generated for the quality of their logic rather than drawn as pictures, so treat any resemblance as a bonus.

Is a nonogram the same as Picross?

Yes — Picross, Griddlers, Hanjie and paint-by-numbers are all names for this puzzle.