Hidden Single sudoku technique

A hidden single turns the naked single around. Instead of asking "what fits in this cell?", you ask "where can this digit go in this row, column or box?". When the answer is a single cell, the digit belongs there, no matter how many other candidates that cell seems to have. The single is "hidden" because the cell itself still looks undecided until you check the unit.

How to find a hidden single

1. Choose a unit — one row, one column, or one box — and a digit missing from it.
2. Mark every cell in that unit where the digit could still legally go.
3. One possible cell means you have found it: place the digit there.
4. Digits that already appear six or seven times in the grid are the best hunting ground, since they have few homes left.

The pattern on the grid

Worked example

The centre box needs a 4. Row 4 already has a 4 at r4c9, striking out r4c4–r4c6; row 6 has one at r6c1, striking r6c4–r6c6; and column 5 carries a 4 at r1c5, removing r5c5. The only survivor in the box is r5c6 — the 4 goes there, whatever else that cell seemed to allow.

When it fails

The classic slip is checking two of the three unit types and stopping. A digit that looks placeable in two cells of a box often loses one of them to a column you have not scanned yet.

Puzzles that need it

These grids come from rated games on this site — our solver confirmed that hidden single was part of the shortest solving path for each one.

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Related techniques

• Naked Single
• Hidden Pair
• Claiming

All solving techniques