Hidden Single
hiddenSingle
LevelNot seen in a hint yet
How it works
If, inside one line (row or column) or one box, a digit can go in only a single cell, that digit is confirmed there no matter how many other candidates that cell still holds.
Where a Naked Single asks "what goes in this cell?", this asks "where does this digit go?". That is why it resolves cells that still look undecided — the answer is hidden behind the other candidates.
Worked example
3
2
7
5
9
8
4
1
6
1
5
3
6
4
2
6
4
7
2
1
5
8
4
1
7
5
3
9
6
2
5
7
6
1
2
3
2
9
3
5
7
1
7
2
3
4
6
8
2
6
7
1
6
5
7
2
● evidence✕ eliminated★ confirmed
Find where 2 can go in row 3. The cells still empty are R3C4, R3C5, R3C6, R3C7, R3C8.
Of those, R3C4, R3C5, R3C7, R3C8 already see a 2 in their own row, column or box (marked ●). The same digit cannot appear twice in one house, so those cells are ruled out (✕).
Only R3C6 is left — 2 goes here (★).
Examples I encountered
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