The overlap method finds cells that must be filled even when a clue block can still move along its line. Place the same block as early as legally possible and as late as legally possible. Any cells covered by that block in both extreme placements are forced filled.
This is one of the most important Nonogram techniques because it turns partial certainty into usable cells without guessing the block's exact position.
A simple overlap example
Take a line of 10 cells with a single clue 7.
The earliest placement fills cells 1–7:
■■■■■■■???
The latest placement fills cells 4–10:
???■■■■■■■
Cells 4–7 are covered in both placements, so those four cells must be filled:
???■■■■???
The outer cells remain unknown because the block can shift across them.
Why the extreme placements are enough
For one unconstrained block, every legal placement lies between its earliest and latest positions. A cell inside the intersection of those extremes is covered no matter where the block slides within that range.
That makes the cell forced.
For a single clue of length k on a line of length n, an initial overlap exists when the block is longer than half the line. But you do not need to memorize a formula to use the method: compare the two extreme placements directly.
Overlap becomes stronger as the available space shrinks
Existing empty cells can reduce the segment in which a block may fit.
Suppose a 10-cell line has clue 5, and cells 1 and 10 are already confirmed empty. The block must fit somewhere inside cells 2–9, an eight-cell segment.
Its earliest legal placement inside that segment is cells 2–6. Its latest is cells 5–9. Therefore cells 5–6 are forced filled.
The clue did not change. The new empty information reduced its placement range and created overlap.
Step-by-step overlap process
For a block you can identify:
- find its earliest legal placement under the current line constraints;
- find its latest legal placement;
- compare the cells occupied by that same clue block in both placements;
- fill the shared cells;
- leave cells that differ between the placements unresolved unless another deduction proves them;
- re-scan crossing lines after every new fill.
That “same clue block” condition becomes crucial when a line contains several clues.
Overlap does not mean “fill everything between two known cells”
The technique is about legal placements, not visual proximity.
Two confirmed filled cells may:
- belong to the same clue block;
- belong to different clue blocks;
- still have ambiguous ownership.
Before extending or joining them, you must respect the clue sequence and block sizes.
How to spot overlap quickly
Look first at:
- large clues relative to the available line length;
- lines with little slack;
- blocks constrained by an edge;
- blocks trapped between confirmed empty cells;
- lines where previous deductions have shortened the block's legal range.
Overlap is not only an opening technique. It can reappear repeatedly as the line becomes more constrained.
Common mistakes
Comparing arbitrary placements
Use legal extreme placements under the current known cell states. A placement that crosses a confirmed empty cell or fails to include a required filled cell is not valid evidence.
Overlapping different clue blocks
On multi-clue lines, a cell may appear filled in both a left-packed and right-packed picture but be occupied by different clue blocks. That does not prove the cell is filled. G-025 treats this explicitly.
Filling the whole range of the block
The block's reach is the union of cells it might occupy. Overlap is the intersection of cells it must occupy. Those are opposite ideas and should not be confused.
Stopping after one pass
Every new X or filled cell can tighten the extreme placements. Recalculate overlap when the line changes.
Overlap and minimum span
Minimum span tells you how tightly the full clue sequence fits. Slack measures how much placement freedom remains.
Low-slack lines are excellent overlap candidates because their blocks cannot move very far.
What to learn next
Once single-block overlap is clear, move to multiple-clue overlap. The idea is similar, but you must track each clue block separately so that cells occupied by different blocks are not mistaken for forced overlap.
FAQ
Is overlap only for one-clue lines?
No. It also works with multiple clues, but each clue block must be compared with its own earliest and latest position.
Is overlap the same as guessing the most likely position?
No. Overlap marks only cells present in every legal extreme placement of the same block. It does not choose one possible position.
What is “Simple Boxes”?
It is a historical/community label commonly used for overlap-style forced fills. VeyraPlay uses overlap as the canonical descriptive term.