Naked Single
A Naked Single is a cell where only one digit is possible, because all the other eight digits are already in the cell's row, column or box.
Learn the techniqueHidden Single
A Hidden Single is a digit that only fits in one cell within a box, row or column. This is true even if the cell could also take other digits.
See the technique in practice
Work through the examples step by step. Each step explains what you see on the puzzle and why the conclusion holds.
Example:
- We look at row 3. The question we ask is simple: in which of the empty cells can digit 5 go?
- Digit 5 is already in the green cells. Follow the highlighted lines into row 3. Where a line hits an empty cell, the digit cannot go there again, and the cell is crossed out. A green 5-digit also blocks the empty cells in its own box, so the cells in the shaded box are crossed out the same way. Then only the cell with the ring around it gets a pass.
- Only one cell remains, namely row 3, column 7.
- Digit 5 must go somewhere in row 3. Since only this cell can take it, it must go here. This is called a Hidden Single.
How to recognize the pattern
A Hidden Single is harder to spot than a Naked Single, because the cell often has several candidates. The key is to switch perspective: instead of asking what can go in a cell, you ask where a specific digit can go in a unit. When the answer is only one cell, you have found a Hidden Single.
Search systematically for digits that already appear many times on the puzzle. If a digit has seven or eight occurrences, the remaining placements are often forced. Boxes are the best place to start, because they are compact and quick to scan.
Step-by-step procedure
- Choose a digit that already appears many times on the puzzle.
- Choose a box, row or column that is missing the digit.
- Mark which empty cells in the unit can take the digit, that is, the cells that are not blocked by the same digit in their own row, column or box.
- If only one possible cell remains, the digit must go there. Write it in.
Common mistakes
- Rejecting a cell because it has many candidates. A Hidden Single does not require the cell to have only one possible digit, just that the digit does not fit anywhere else in the unit.
- Checking only rows. Hidden Singles show up equally often in columns and boxes, so vary which unit you scan.
- Missing a blocking occurrence. One missed occurrence of the digit in a crossing row or column can make you find two possible cells when there is really only one.
When do you need the technique?
The easiest sudokus are solved with two techniques that both build on the same rule: each digit can only appear once in each row, column and box. If you learn these two, you can solve all puzzles on Easy level.
Try it on your own puzzle
Enter your puzzle in the Sudoku Solver and it will find the next step and explain the technique behind it.
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