15×15 nonograms

Fifteen squares a side is the classic size: big enough for a real picture, small enough to finish in one sitting. Pick a difficulty and start.

What changes at 15×15

On a 5×5 or 10×10, a few big clues often break the puzzle open. At 15×15 the clues are shorter compared with the line, so first-pass overlaps give you less, and you win by moving back and forth between rows and columns. This is the size where the thick guide lines every five squares start to earn their keep: they let you count positions at a glance instead of square by square.

Techniques that matter. With 225 squares and thirty lines, the crosses you place do real work: they split a 15-wide line into segments, and a segment shorter than the smallest unplaced run in that line must be empty. Keep track of the longest run still to place in each line, because that one number tells you which gaps you can cross out.

Tips for 15×15

1. Scan for big sums first. Any line whose runs plus gaps come within two or three of 15 gives you overlap squares straight away.
2. Count from the guides. "Square 11" is the first square past the second guide, which is faster and less error-prone than counting from the edge.
3. Cross aggressively. Crosses on a 15-wide line split it into short segments, and short segments are where techniques 3 and 5 do their work.

Expert boards can't be finished with line-by-line reasoning alone. You'll need lookahead (see How to play).