Outcome
You can find a digit that has only one possible position in a row, column, or box, even when the target cell shows other candidates.
Intro copy
A cell can still show several candidates while one digit is already forced there.
Change the question from “What fits in this cell?” to “Where else could this digit go?”
Step 1 — Explain
Show one row with candidate 6 appearing in exactly one cell. That cell also contains other candidates.
Copy:
6 is hidden among other notes, but in this row it has only one possible home.
Step 2 — Guided example
Prompt
Row 3 still needs a 6. Tap every cell where 6 could legally go.
Hint 1
Check each empty cell against its column and box.
Hint 2
Only one cell survives those restrictions.
Correct feedback
Correct. That cell still had other candidates, but it was the only place for 6 in the row.
Incorrect feedback
That cell is blocked from containing 6. Check its crossing unit.
Step 3 — Try it
Exercise spec
A box Hidden Single found through crosshatching. No candidate filter unless requested as a hint.
Correct feedback
Correct. Crosshatching left one position for the digit, so you found a Hidden Single in the box.
Incorrect feedback
Follow the existing copies of the target digit through the crossing rows and columns.
Step 4 — Practice
Five exercises:
- row Hidden Single;
- column Hidden Single;
- box Hidden Single;
- dense-candidate row using candidate filter;
- mixed state with one Naked Single distractor — learner is specifically asked to find the Hidden Single.
Checkpoint
Prompt:
Find a digit that has exactly one possible position in any row, column, or box.
The app should accept any valid Hidden Single if the state contains more than one.
Completion
At least one correct Hidden Single in each unit type plus two mixed exercises.
Next
Beginner Practice — combine scanning, crosshatching, Naked Singles, and Hidden Singles without being told which technique to use.
Reference
Read the full Hidden Single Guide for recognition methods and the difference from Naked Singles.