Nonograms for Beginners: How to Read the Clues

A nonogram looks like a crossword that lost its words. The clues are easier to read than they seem, and a 5 by 5 grid is the ideal place to learn.

A nonogram is a grid with numbers along the top and down the side, and no other instructions. You shade some squares, leave others empty, and if you get it right a small picture or pattern appears. You may also know it as picross or griddlers. Whatever the name, the logic is the same, and the five by five Nonogram on GridPencil is small enough to learn it properly.

What the numbers mean

Each row and each column has its own clue. The clue lists the runs of shaded squares in that line, in order.

• A clue of 3 means: somewhere in this line there is one unbroken run of three shaded squares. Everything else in the line is empty.
• A clue of 1 1 means: two separate shaded squares, with at least one empty square between them.
• A clue of 2 1 means: a run of two, then a gap, then a single square. The two comes first, reading left to right or top to bottom.
• A clue of 0 means the line is completely empty.

That is all there is to the rules. The rest is working out where the runs sit.

Start with the lines that have no freedom

In a line of five squares, some clues can only fit one way.

A 5 fills the whole line. Shade it all.

Clues that use up the full width are just as certain. Count the shaded squares and add one for each gap. For 3 1, that is three plus one plus a gap of one: five. It fits exactly, so you can fill it in at once. The same is true of 1 3, 2 2 and 1 1 1.

A 0 is also a gift. Put a cross in every square of that line. In our version you tap once to shade a square and a second time to mark it with a cross.

The overlap trick

This is the single most useful idea in nonograms, and it is easy to see on a small grid.

Take a row with the clue 4. The run of four can sit hard against the left edge, covering squares one to four. Or it can sit against the right edge, covering squares two to five. Either way, squares two, three and four are shaded. You do not know yet about the ends, but the middle three are certain.

Now a clue of 3. Pushed left, it covers squares one to three. Pushed right, it covers three to five. Only square three is in both. So a 3 always shades the centre square, and that one square is often enough to get a column started.

The general rule: imagine the runs pushed as far one way as they will go, then as far the other way. Any square shaded both times is definitely shaded.

Crosses matter as much as shading

Beginners shade the squares they are sure about and ignore the ones they know are empty. That throws away half the information.

Suppose a row has the clue 2 and you have already shaded two squares side by side in it. The run is complete. Every other square in that row is empty, so mark them with crosses. Now each of those crosses shortens the space available in its column, and a column clue that had two possible positions may be down to one.

Go back and forth

Solve what you can in the rows. Then read the columns, using what the rows have told you. Then the rows again. Each pass is easier than the one before, because every settled square is a new fact for the line that crosses it.

On a five by five grid, three passes is usually enough.

A common trap

In our version, a clue turns green when its line matches. That is a helpful check, but it is not proof. A row can match its own clue and still be wrong, because the shaded squares are in the right pattern at the wrong position for the columns. If a column refuses to work, look at the green rows crossing it before you look anywhere else.

Where to go next

Larger nonograms, ten by ten and beyond, use nothing you have not just read. The overlap trick does most of the work, the crosses do the rest, and the only new ingredient is patience.

If you like this kind of reasoning, where each line constrains the ones that cross it, try Binary Grid or Sudoku Six next. They are the closest relatives among our HTML5 games, and the back-and-forth habit you learn here carries straight over.