Hidden 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.
Learn the techniqueNaked 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.
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 the highlighted cell at row 5, column 5. To figure out which digit fits there, we check the row, column and box that the cell belongs to.
- First, we write all digits from 1 to 9 as small candidate notes in the cell. Then we remove the digits that turn out to be impossible.
- Each digit can only appear once in each row, column and box. The green cells show the digits already in place around our cell, so we remove those digits from the notes.
- Only digit 5 remains. All the other digits are ruled out, so 5 must go in row 5, column 5. A cell with only one possible candidate is called a Naked Single.
How to recognize the pattern
A Naked Single hides in cells surrounded by many filled neighbors. Look for rows, columns and boxes that are almost done, because that is where you have the best chance of finding a cell with only one possible digit left. If you have candidate notes on, the pattern is even easier to see: a cell with only one note is a Naked Single.
Many underestimate how far this technique reaches. On Easy level puzzles you can usually solve the entire puzzle just by finding Naked and Hidden Singles over and over again, because each placement opens up new Singles elsewhere.
Step-by-step procedure
- Choose an empty cell in an area with many filled cells.
- Go through digits 1 to 9 and remove each digit that is already in the cell's row, column or box.
- If you are left with exactly one digit, it must go in the cell. Write it in.
- Then check the neighboring cells again, because the placement may have created new Naked Singles.
Common mistakes
- Forgetting the box. Row and column are easy to check, but it is often the box that removes the last candidate, so always check all three units.
- Relying on old candidate notes. After each placement, the notes in the cell's row, column and box must be updated, otherwise you will see Singles that no longer match.
- Mixing up Naked and Hidden Single. A Naked Single is about a cell having only one possible digit, while a Hidden Single is about a digit having only one possible cell.
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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