Nonogram overlap explained, step by step

Read the clue before filling anything

A clue of 4 means one uninterrupted run of four filled squares. It does not mean four filled squares scattered anywhere in the line. If the clue is 2, 1, there must be a run of two, at least one empty square, then a run of one, in that order.

Rows and columns follow the same rule. For this first example, consider a completely undecided row of six squares with the single clue 4.

Try the leftmost and rightmost positions

The run can start at square 1, 2 or 3. Pushed left, it fills squares 1–4. Pushed right, it fills squares 3–6. Squares 3 and 4 appear in both extremes, and in the middle position too.

Fill squares 3 and 4. Leave 1, 2, 5 and 6 undecided: they are not guaranteed empty. We know part of the run, but not yet where it starts or ends.

Use the shortcut carefully

For a single run in an otherwise empty line, double the run length and subtract the line length. A positive answer is the number of guaranteed filled squares in the middle. Here, 2 × 4 − 6 = 2.

A run of 3 in a line of 6 gives no overlap yet. That does not mean the line is useless; its crossing clues may narrow the possibilities later. Do not apply the simple shortcut to several runs as though they were one block.

Remember the gaps between runs

For a five-square line with clues 2, 2, the two runs take four squares and their compulsory gap takes the fifth. There is only one arrangement: filled, filled, empty, filled, filled.

In a longer line, extra empty squares may occur before, between or after the runs. You must account for the minimum gaps when comparing possible positions.

Let the columns return the favour

After filling an overlap in a row, inspect the columns that cross those squares. Their possible run positions may now agree on more filled squares or on definite empties. Mark an × only when an empty square is certain.

Return to the row when those columns give you new information. This back-and-forth between lines is the main rhythm of a Nonogram solve. It is more reliable than guessing what picture the pattern might become.

• Keep undecided and definitely empty squares distinct.
• Do not add an empty square beside a partial run until you know the run is complete.
• On HubbardGames, Fill and Empty brushes make longer lines easier to mark.