Lesson 3 of 9
Naked single
One square, one surviving candidate. The most direct placement in Sudoku.
What this grid does
- Starting digits
- 25 of 81
- Deductions to solve
- 59
- Uses of Naked single
- 2
- Squares placed by scanning
- 56
- First needed at
- deduction 16 (R7C1, R2C1, R3C1, R7C4, R7C5, R5C1, R7C6, R7C7, R7C8, R8C1, R7C9, R9C1, R8C2, R8C3, R9C2 and R9C3, on the 2)
- Also calls for
- last free cell, cross-hatching, hidden single and naked pairs
Measured on this grid by the same solver the hint button uses — how these puzzles are chosen.
Why this one is worth drilling
About 90 in every 100 puzzles need it, so you meet it constantly and can learn it simply by playing. That figure is measured rather than guessed: 90% of solvable 24-clue grids over 300 digs.
A naked single is never the thing you are stuck on; it is the thing you find once your notes are right. If none is showing, either the notes are stale — a digit was placed and its candidates never erased — or the position genuinely needs a unit-based argument instead.
Where it sits in the nine
Third, and the first that needs written candidates, which is why it comes after the two that do not. It is the pay-off for notes rather than a technique you hunt for — annotate correctly and naked singles appear on their own. Treat this lesson as practice at keeping notes accurate under pressure, because lessons five onwards are unusable without that.
Working it
This grid rewards annotating one unit at a time rather than all eighty-one squares. Fill the candidates for a single box, read it, act on it, then update. A fully annotated grid goes stale faster than you can use it, and a stale note is worse than no note because you will trust it. A useful check: if a square shows one candidate but placing it feels wrong, re-derive that square from scratch rather than trusting the note. Most apparent contradictions at this level are a stale candidate from several placements ago, not a mistake in the puzzle — and every grid on this site is verified to have exactly one solution, so the puzzle is not what is wrong.
The full explanation of naked single covers the reasoning with diagrams.
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