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Sudoku technique · Foundations · first needed in Calm puzzles
Hidden single
One digit has just one remaining location in a house, even if that cell has several pencil marks.
How to spot it
Follow one digit through an entire row, column, or box.
The common mistake
Do not confuse one option in a cell with one location for a digit in a house.
Learn this first
Candidates and the row / column / box rules.
A worked example
This position comes from SudokuByte’s lesson on the Hidden 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
12567
9
1456
23
23
235
13467
8
23457
2359
258
3589
23489
23689
1
34689
7
23489
4
6
7
5
2389
2389
1389
129
2389
1239
128
1389
23489
236789
234689
5
12469
23489
1379
147
1349
13489
5
3489
2
49
6
15679
1457
2
1489
689
4689
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
12567
9
1456
23
23
235
13467
8
23457
2359
258
3589
23489
23689
1
34689
7
23489
4
6
7
5
2389
2389
1389
129
2389
1239
128
1389
23489
236789
234689
5
12469
23489
1379
147
1349
13489
5
3489
2
49
6
15679
1457
2
1489
689
4689
4789
3
45789
8
45
34569
2349
2369
7
49
459
1
Where to look
Follow one digit through an entire row, column, or box.One digit has just one remaining location in a house, even if that cell has several pencil marks.
Read the evidence in this position
Follow 7 through row 6. Only r6c5 still has that candidate.Why the deduction must follow
The house must contain 7 exactly once. Other candidates in r6c5 cannot change the fact that no other cell can supply this 7.Apply the result
Place 7 in r6c5.
Further reading: hodoku.sourceforge.net