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Sudoku technique · Foundations · first needed in Calm puzzles
Naked single
One cell has just one candidate left. Place that digit.
How to spot it
Inspect small candidate lists, not just rows with lots of filled cells.
The common mistake
A single pencil mark is proof only when the candidate list is complete and its earlier reductions were valid.
Learn this first
Candidates and the row / column / box rules.
A worked example
This position comes from SudokuByte’s lesson on the Naked single. Gold cells hold the evidence; green cells are the pattern’s ends.
256
2458
4568
7
1
23589
3469
24569
23459
1257
3
158
6
4
2589
179
1259
2579
1567
9
1456
23
23
5
1467
8
457
2359
258
3589
2349
23689
1
34689
7
23489
4
6
7
5
2389
23
1389
129
2389
239
1
389
2349
7
2346
5
2469
23489
139
7
1349
8
5
34
2
49
6
569
45
2
1
69
46
4789
3
45789
8
45
34569
2349
2369
7
49
459
1
256
2458
4568
7
1
23589
3469
24569
23459
1257
3
158
6
4
2589
179
1259
2579
1567
9
1456
23
23
5
1467
8
457
2359
258
3589
2349
23689
1
34689
7
23489
4
6
7
5
2389
23
1389
129
2389
239
1
389
2349
7
2346
5
2469
23489
139
7
1349
8
5
34
2
49
6
569
45
2
1
69
46
4789
3
45789
8
45
34569
2349
2369
7
49
459
1
Where to look
Inspect small candidate lists, not just rows with lots of filled cells.One cell has just one candidate left. Place that digit.
Read the evidence in this position
r3c6 has just one remaining candidate: 5.Why the deduction must follow
There is no alternative left for r3c6. Candidate reductions can reveal a placement without any trial or guess.Apply the result
Place 5 in r3c6.
Further reading: hodoku.sourceforge.net