An exact-fit line is a row or column where the clue blocks, together with the mandatory empty cells between them, use every available cell. When that happens, there is no placement freedom left: every filled cell and every separator can be marked immediately.
This is one of the strongest beginner deductions because it can solve a whole line in a single step.
The exact-fit rule
For a monochrome Nonogram, consecutive clue blocks must be separated by at least one empty cell.
For clues 4 5, the shortest possible arrangement is:
■■■■ × ■■■■■
The blocks use 9 filled cells and need 1 mandatory gap, so their minimum span is 10.
If the line itself is also 10 cells long, that arrangement is forced.
There is no room to shift either block left or right. The line is completely determined.
How to calculate whether a line is an exact fit
Add all clue values, then add one mandatory gap for every pair of consecutive clues.
For 3 4 6:
- filled cells:
3 + 4 + 6 = 13; - mandatory gaps:
2; - minimum span:
15.
So on a 15-cell line, the only valid arrangement is:
■■■ × ■■■■ × ■■■■■■
For k clue blocks, there are k - 1 mandatory gaps between them. A single clue therefore has no internal mandatory gap.
Exact fit is more than “a clue as long as the line”
A clue equal to the entire line is the simplest exact fit. A clue of 10 on a ten-cell row obviously fills all ten cells.
But exact fit is much more useful than that. Multiple clues can also consume the entire line once their required separators are included.
For example, a 12-cell line with clues 3 3 4 has:
- 10 filled cells;
- 2 required gaps;
- minimum span 12.
The two empty separators are just as forced as the filled blocks.
This matters because beginners often see the block lengths but forget that the spaces between monochrome blocks are part of the constraint.
Exact fits can appear later inside a segment
The technique is not limited to a fresh row or column.
Confirmed empty cells can split a line into smaller open segments. If you know which unresolved clues must fit inside one of those segments, compare their minimum span with the segment length.
Suppose a segment is eight cells long and must contain clues 3 4.
Their minimum span is:
3 + 1 + 4 = 8
So the segment is an exact fit:
■■■ × ■■■■
This is one reason X marks are so valuable. They can turn a loose full line into a tightly constrained smaller problem.
Step-by-step recognition process
When checking a line:
- add the clue values;
- count one mandatory gap between every pair of clues;
- compare that minimum span with the usable line or segment length;
- if the two lengths are equal, place every block in clue order;
- mark every mandatory gap empty;
- propagate those new cells into the crossing lines.
You do not need to test alternative arrangements because there are none.
Why exact fit is logically safe
The clues define both block size and block order. The monochrome rules also require separation between neighboring blocks.
If those minimum requirements already consume the whole available space, moving any block would require either:
- shortening a clue block;
- changing clue order;
- removing a mandatory separator; or
- extending beyond the available cells.
All four violate the puzzle rules. The compact arrangement is therefore the only valid one.
Common mistakes
Forgetting the separators
For clues 4 5, the minimum span is not 9. It is 10 because the two blocks need one empty cell between them.
Adding gaps outside the first and last block
The minimum-span calculation includes gaps between consecutive blocks. It does not automatically require an empty cell before the first block or after the last block.
Treating an almost exact fit as an exact fit
If the line is 11 cells long and the minimum span is 10, there is one cell of slack. The blocks can still shift. You may get forced cells from overlap, but you cannot fill the entire line as if it were exact.
Applying a segment exact fit before clue assignment is known
A small segment may have the right size for a clue, but you must first know that the clue can legally belong to that segment. Segment capacity alone does not always prove clue identity.
Exact fit versus overlap
Exact fit removes all placement freedom. Overlap works when some freedom remains but certain cells are still covered in every legal placement.
A useful progression is:
- minimum span equals available length → exact fit;
- minimum span is slightly smaller → check overlap;
- plenty of slack remains → use existing cell states, edges, or cross-line information to reduce the possibilities.
What to learn next
Once exact fits are automatic, learn to measure minimum span and slack. That lets you identify lines that are close to exact fits and therefore likely to produce strong overlap deductions.
FAQ
Does an exact-fit line always solve the entire row or column?
Yes, if all clues for that available line are known to belong there and their minimum span equals the available length. Every block and required internal separator is then fixed.
Can a line with several clues be an exact fit?
Yes. In fact, multi-clue exact fits are common. Remember to include the mandatory empty cell between each consecutive pair of monochrome blocks.
Can exact fit be used after the puzzle has already started?
Yes. Confirmed empty cells can create smaller open segments, and assigned clues may exactly fill one of those segments even when the original full line was not an exact fit.