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Nonogram Small Gaps: Too Short for Any Block

Small gaps is a quick rule: a stretch of open squares between two crosses (or a cross and the edge), with nothing filled in it, that’s shorter than the smallest block in the clue, can’t hold a block, so it’s empty. It’s the least-used of the six, but on big puzzles it saves you from packing a whole line to find one square.

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This is the fifth of the Nonogram techniques.

The rule

  1. Find the smallest number in the line’s clue.
  2. Look for stretches of open squares between crosses, or between a cross and the edge, with nothing filled.
  3. Cross out any stretch shorter than the smallest number.
Small gaps: too short for any block A row with the clue 2 3 and squares 2 and 5 crossed out. Square 1 is a gap of one square, too short for even the 2, so it is crossed out. 2 3 2 3 before after
Square 1 is a gap of one: too short for even the 2.

In a real puzzle

In the 10×10 every technique page follows, row 1 (a single 2) gets a small gap at step 40. Square 7 is a one-square gap between the crosses at squares 6 and 8. A 2 can’t fit, so square 7 is crossed out.

row 1, before and after row 1 with the clue 2, before and after this step. 2 2 before after
Row 1: square 7 is a gap of one, too short for the 2.
small gaps in the 10 by 10 puzzle The puzzle at step 40 of 49, just after this technique settles row 1 (clue 2). The squares it settled are highlighted. 2 1 3 2 6 2 1 2 2 3 1 2 4 9 5 3 2 3 3 6 6 1 4 1 6 3 5 5 2 2 1 2 1 2 3 1 2 3
The puzzle at step 40, with row 1 highlighted.

How often you’ll need it

Not often. On 100 of the game’s puzzles per size, small gaps was needed in none of the 5×5s or 10×10s, and in 21 of the 15×15s. The other techniques usually find the same squares first; small gaps matters when they don’t, on long lines broken up by crosses.

Next steps

Questions

Why only the smallest block?

Because it’s quick and always safe: if even the smallest block can’t fit, none can. A gap that only the smallest block fits might still be empty; that needs more thought.

Does the gap have to be empty already?

Yes, apart from being open. A gap with a filled square in it has to hold a block, so if no block fits, something is wrong earlier in the puzzle.