Outcome
You can prove that a cell has exactly one remaining candidate and place it with confidence.
Intro copy
Sometimes a cell has already been solved by elimination — it just has not been filled yet.
If only one candidate survives, place it.
Step 1 — Explain
Show a target cell with candidate set {8}.
Copy:
A Naked Single belongs to the cell itself: every digit except one is blocked by its row, column, or box.
Step 2 — Guided example
Prompt
What can go in r4c7?Hint 1
Check the digits already present in row 4.
Hint 2
Now add the digits blocked by column 7 and the box.
Correct feedback
Good. 8 is the only legal candidate, so r4c7 = 8.Incorrect feedback
That digit is blocked by one of the cell's peers. Check the highlighted row, column, and box.
Step 3 — Try it
Exercise spec
Target cell displays three candidates initially. Learner performs one already-explained candidate elimination from a recent solved move, reducing the cell to one candidate, then places it.
Correct feedback
Nice. Removing that impossible candidate left one possibility, which turned the cell into a Naked Single.
Incorrect feedback
Update the notes first. One candidate is no longer legal after the previous placement.
Step 4 — Practice
Five exercises:
- three with full candidates;
- one after a candidate update;
- one with notes hidden, requiring peer inspection.
Checkpoint
Mixed state with several low-candidate cells. Only one is a true Naked Single.
Prompt:
Find the cell you can solve from its own candidates alone.
Completion
Five correct Naked Single placements. The final no-notes exercise must be solved without entering an unproven digit.
Next
Hidden Singles — learn the other kind of Single, where a digit has one position in a unit even though the cell has several candidates.
Reference
Read the full Naked Single Guide for comparison with Hidden Singles and last-missing-digit placements.