Crosshatching is a focused scanning method for one digit and one box.
Existing copies of the target digit block cells along their rows and columns. If those crossing restrictions leave only one legal cell inside the box, the digit is forced there.
Crosshatching is the search method. The resulting forced placement is usually a Hidden Single.
How crosshatching works
Suppose you want to place digit 6 in the center box.
- Find existing 6s in rows that pass through the center box.
- Those rows cannot contain another 6, so cells on those rows inside the box are blocked.
- Find existing 6s in columns that pass through the center box.
- Those columns also block cells inside the box.
- If exactly one cell remains, that cell must contain 6.
Nothing new has been added to the Sudoku rules. Crosshatching is simply a visual way to apply the “one of each digit per row, column, and box” constraint.
Step-by-step example
Why this is a Hidden Single
A Hidden Single means a digit can appear in only one cell of a unit.
Crosshatching is one way to find that situation visually.
The target cell itself may still have several candidates if you write all notes. What matters is that the chosen digit has no other possible position in the box.
For example, the final cell might show {2, 6, 9}. It is still solved as 6 if every other cell in the box is unable to contain 6.
That is why it is “hidden”: 6 is not necessarily the cell's only candidate.
Crosshatching across horizontal bands
A band is a group of three horizontally aligned boxes.
If two boxes in a band already contain digit 4 in two different rows, those rows are blocked in the third box. The remaining row becomes the only row where 4 can appear in that box.
A crossing column may then reduce that row to one exact cell.
This is a common visual pattern because the three-box band lets you compare the same digit quickly.
Crosshatching across vertical stacks
The same idea works vertically.
A stack is a group of three vertically aligned boxes.
Existing copies of a digit in two columns can restrict the third box to a remaining column, and crossing row constraints can identify the exact cell.
The logic is symmetric: rows and columns simply exchange roles.
Crosshatching does not always give an immediate placement
Sometimes the scan shows that a digit can occupy two cells in the target box, both on the same row.
You cannot place the digit yet.
But you have learned something useful: within that box, the digit is locked to that row.
That observation is the foundation of Pointing / Locked Candidates, an intermediate technique that can eliminate the digit from the rest of the row outside the box.
This makes crosshatching a natural bridge between beginner scanning and intermediate candidate logic.
A fast crosshatching routine
Pick one digit
Do not scan all digits at once. Start with a digit that already appears several times in the grid.
Move box by box
For the chosen digit, identify boxes that do not yet contain it.
Use visible blockers
Trace rows and columns from placed copies of the digit.
Stop when the box is resolved
If one cell remains, place the digit. If two or more remain, keep the information and move on.
Re-scan after placement
The new digit becomes another blocker for neighbouring boxes and may create the next crosshatching opportunity immediately.
Common crosshatching mistakes
Blocking from the wrong digit
Crosshatching is performed for one target digit at a time. Other solved digits matter for cell legality, but the row/column lines you are tracing are based on existing copies of the target digit.
Treating two remaining cells as a guess
If two cells remain, the digit is not placed yet. The restriction may still be useful later as Locked Candidates.
Looking only inside the target box
The blocking information comes from rows and columns outside the box as well.
Assuming the final cell must have one candidate total
Crosshatching often finds a Hidden Single. The cell can still contain other notes; the target digit is forced because it has one location in the unit.
FAQ
Is crosshatching the same as a Hidden Single?
Not exactly. A Hidden Single is the logical result: a digit has one possible cell in a unit. Crosshatching is a visual search method that can reveal that result.
Can crosshatching be used on rows and columns, not only boxes?
The same candidate-restriction idea can be viewed from any unit, but the term is commonly taught through rows/columns crossing a box. VeyraPlay will use that presentation for consistency.
What if crosshatching leaves two cells?
Do not choose between them. Record the restriction. If both cells lie on one row or column inside the box, that may create a Locked Candidates elimination elsewhere.
Do I need candidates visible?
No. Crosshatching is particularly useful as a visual technique before full candidate notation.
What to learn next
Continue to Hidden Singles to understand the deduction crosshatching usually reveals. Then learn Locked Candidates to use the same box-line interaction when a digit is restricted but not yet placeable.