Outcome
You can track one digit through crossing rows and columns until only one position remains inside a box.
Intro copy
Crosshatching is a focused one-digit scan.
Choose a digit, use existing copies to block rows and columns, and watch the remaining space inside the box shrink.
Step 1 — Explain
Display a target box without digit 6.
Copy:
A row containing 6 cannot contain another 6. The same is true for a column. Use those crossing lines to remove possible cells inside the box.
Step 2 — Guided example
Prompt
We need a 6 in this box. Tap the existing 6s that block positions here.
Hint 1
Start with the 6 in the row crossing the top of the box.
Hint 2
Now check the column of the other nearby 6.
Correct feedback
Great. Only one cell survives the crossed rows and columns, so 6 must go there.
Incorrect feedback
That cell lies on a row or column that already contains 6. Follow the blocker back to the solved 6.
Step 3 — Try it
Exercise spec
A second box and a different digit. No blocking overlays initially; user can reveal one overlay as a hint.
Correct feedback
Correct. You used the existing copies of the digit to leave one legal position.
Incorrect feedback
Check whether that row or column already contains the target digit.
Step 4 — Practice
Three states:
- horizontal-band crosshatching;
- vertical-stack crosshatching;
- a state where two cells remain — learner must choose “not enough information yet” rather than guess.
Checkpoint
Prompt:
Find the box where digit 4 has only one valid position, then place it.
No preselected target box.
Completion
Three placement tasks plus the “two cells remain” non-placement task must be correct. Checkpoint completed with at most one hint.
Next
Naked Singles — switch from “where can this digit go?” to “what can go in this cell?”
Reference
Read Crosshatching in Sudoku for the relationship between crosshatching, Hidden Singles, and future Locked Candidates.