Edge anchoring uses the hard boundary at the start or end of a row or column to reduce where an identified clue block can fit. When a confirmed filled cell lies close enough to an edge, a block that must include that cell may be forced to extend inward through additional cells.
The grid border acts like a constraint: the block cannot extend beyond it.
A terminology note about “edge logic”
The phrase edge logic is not standardized across Nonogram sources. Some tutorials use it for this single-line border effect. Other advanced communities use edge logic or boundary logic for reasoning that starts from an edge clue and tests its consequences across several perpendicular lines.
VeyraPlay therefore calls the technique on this page edge anchoring: one identified block is constrained by the physical start or end of its own line. The broader multi-line meaning is covered separately in Multi-Line Reasoning.
Example: a five-cell block near the left edge
Take a 10-cell line with a single clue 5. Suppose cell 2 is already confirmed filled.
Because there is only one block, that filled cell must belong to the five-cell clue.
A length-5 block containing cell 2 has only two legal placements:
- cells 1–5;
- cells 2–6.
Cells 2–5 are filled in both placements, so they are forced.
The left border prevents the block from sliding any farther left, which creates a large shared region.
The same reasoning works from the right edge
Now take a 10-cell line with clue 4 and a confirmed filled cell at position 9.
The block can occupy:
- cells 6–9; or
- cells 7–10.
Therefore cells 7–9 are forced filled.
You do not need a special right-edge rule. Reverse the line mentally and apply the same placement logic.
Edge anchoring is constrained overlap
At a logical level, edge deductions are closely related to overlap.
The difference is what creates the tight range:
- ordinary overlap may come from a long block in a short line;
- edge anchoring often becomes strong because a confirmed cell anchors the block close to a hard boundary.
The safe proof is still based on legal placements and their shared cells.
Block identity matters on multi-clue lines
Edge anchoring becomes more delicate when a line has several clues.
A filled cell near the left edge is not automatically part of the first clue. You must prove that later clue blocks cannot legally reach it, or otherwise establish which clue owns that filled fragment.
Likewise, a filled cell near the right edge is not automatically part of the final clue.
Clue order helps, but ownership must be justified from the current constraints.
A useful recognition pattern
Ask three questions:
- Which clue block can this confirmed filled cell belong to?
- How far can that block extend toward the nearest edge?
- Across every legal placement that includes the cell, which additional cells are always covered?
Those always-covered cells are safe fills.
What edge anchoring does not let you do
A confirmed cell does not mean “fill clue length cells inward from here.”
For example, if clue 5 has a filled cell at position 4 in a 10-cell line, the block can start at positions 1, 2, 3, or 4. The exact endpoints are still uncertain.
Only the cells common to every legal placement are forced.
Edge anchoring can also create forced empty cells
If a single identified block is constrained near an edge, some distant cells may fall outside its entire legal reach. On a one-clue line, those cells can be marked empty because there is no other block that could occupy them.
That deduction is better understood through block reach, where we distinguish cells a block may occupy from cells it must occupy.
Historical “Glue” terminology
Some older Nonogram technique lists use the name Glue for related situations where a filled cell close to an edge forces more of its block.
VeyraPlay uses edge anchoring for this single-line border deduction and extending confirmed blocks for the related fragment-extension idea. This avoids overloading the phrase edge logic, which some advanced Nonogram sources use for a different multi-line boundary technique that studies consequences in crossing lines.
Step-by-step edge check
When a line contains a confirmed filled cell near a border:
- identify which clue block must contain it;
- list or visualize that block's earliest and latest legal placement while still covering the cell;
- respect the grid boundary and every existing X mark;
- fill the cells shared by all those placements;
- mark distant cells empty only if no other unresolved block can reach them;
- propagate the new information into crossing lines.
Common mistakes
Assuming the nearest clue owns the cell
Clue order does not by itself prove ownership. Check whether another clue block could legally cover the cell.
Extending by the full clue length from a single filled cell
The confirmed cell may be in the middle of the block. Determine all legal placements first.
Treating the border as an empty cell that requires an extra separator
The border stops the block, but no separator is required outside the grid.
Forgetting other blocks when marking unreachable cells
A cell outside one block's reach may still be reachable by another clue block. Mark it empty only when every legal block assignment excludes it.
What to learn next
Edge anchoring naturally leads to extending confirmed blocks, where the same idea is applied to fragments constrained by X marks and other boundaries, and to block reach, which formalizes the full region each clue can still occupy.
FAQ
Is edge anchoring a separate rule of Nonograms?
No. It is a solving technique derived from the normal clue constraints and the fact that blocks cannot extend beyond the line boundary.
Does it work on both rows and columns?
Yes. A line can be read left-to-right or top-to-bottom; the logic is identical.
Is “Glue” the same thing?
The historical label overlaps with these ideas, especially when a known filled cell near a border forces extension. VeyraPlay uses more descriptive terms so the reasoning remains clear in different contexts.