Block Capping
Beginner technique
Block capping is the rule that a completed run of filled cells must be bounded on both sides by empty cells (or the edge of the grid). Once you know a run is complete — i.e., it exactly matches a clue value — you can immediately mark the adjacent cells empty.
When is a run complete?
A run is complete when:
- You've placed exactly as many consecutive filled cells as the clue specifies, AND
- The cells on both ends of the run are either the grid edge or an empty cell (or can be deduced to be empty).
Often block capping is triggered automatically: when simple overlap fills a run to its full length, the cells on either side become cappable.
Example
Clue: 3 in a 7-cell row. Simple overlap has determined cells 3, 4, and 5 are filled. That's 3 cells — matching the clue exactly. Therefore:
- If cells 2 and 6 are unknown, mark them empty.
- If cell 2 is already empty (e.g. it's the left edge), only mark cell 6 empty.
Why it matters
Capping a run creates empty cells, which in turn partition the line into independent segments. Each segment can then be solved with the remaining clues, independently. Block capping is one of the primary ways a nonogram "opens up" as you solve it.
Multi-run interaction
In a line with multiple runs, capping one run can immediately constrain the placement of the next. If clue is 2 3 and the first run of 2 is capped, the run of 3 must start at least 2 positions after the cap.
Related techniques
- Simple overlap — often triggers block capping
- Clue exhaustion — fill completion from a different angle
- Edge deduction — capping at grid boundaries