Naked Single and Hidden Single: Differences and Examples

Naked Single and Hidden Single are two of the most important techniques for starting to solve Sudoku puzzles with a method.

Both let you enter a number with certainty, without guessing. The difference is in how you reach the conclusion: in a Naked Single you look at a cell, while in a Hidden Single you look at a row, column, or block.

Understanding this difference well is essential because many beginners confuse the two techniques. In reality, once the point of observation is clear, they become much easier to recognize.

In this guide, we compare Naked Single and Hidden Single with simple explanations, practical examples, and a method for practicing them together.

Naked Single and Hidden Single have a common goal: finding a certain move.

In a Naked Single, the analyzed cell has only one candidate remaining. The other numbers have been excluded by the grid's constraints or by previous logical eliminations.

In a Hidden Single, instead, a certain number can go in only one position within a row, column, or block. The cell may also contain other candidates, but that number is forced because there are no other valid positions in the area.

The difference is not in the final result, but in the logical path that leads to it.

To understand whether you are using a Naked Single or a Hidden Single, look at where your reasoning starts.

If you start from a cell and discover that it has only one possible candidate, you are using a Naked Single.

If you start from a number and look for its only possible position within a row, column, or block, you are using a Hidden Single.

The guiding question is different:

• Naked Single: “what can go in this cell?”
• Hidden Single: “where can this number go in this area?”

You are using Naked Single reasoning when you analyze a specific cell.

The question to ask yourself is: which numbers can still go in this cell?

To answer, you check the row, column, and block. If all numbers except one are already excluded, then the cell has only one possible candidate.

This type of check is very useful when an area of the grid is almost complete. The more numbers are already present around the cell, the more likely it is that only one possibility remains.

You are using Hidden Single reasoning when you analyze an entire area of the grid.

The question to ask yourself is: where can this number go in this row, column, or block?

For example, if the number 8 is missing from a block, you need to check all empty cells in the block. If the number 8 can be placed in only one of those cells, then you have found a Hidden Single.

In this case, it does not matter if the cell also has other candidates. The point is that the number 8 has no other possible positions in that area.

Imagine an empty cell. Its row contains 1, 2, and 9. Its column contains 3, 5, and 6. Its block contains 4 and 8.

The excluded numbers are therefore 1, 2, 3, 4, 5, 6, 8, and 9. The only remaining number is 7.

This cell must contain 7. It is a Naked Single because the cell has only one possible candidate.

The reasoning starts from the cell and ends in the cell: you look at which numbers are excluded and find the only one remaining.

Now imagine a 3×3 block where the number 5 is not present yet.

There are three empty cells in the block. The first cannot contain 5 because a 5 is already present in the same row. The second cannot contain it because a 5 is already present in the same column. The third, instead, can contain 5 and is the only available position in the block.

At this point, 5 must go in the third cell. Even if that cell had other candidates, 5 would still be forced.

It is a Hidden Single because the number 5 has only one possible position in the block.

In easy Sudoku puzzles, both Naked Single and Hidden Single often appear.

The Naked Single tends to be more immediate when you use candidates, because you can immediately notice a cell with only one possible number. The Hidden Single can be a little less obvious because it requires observing an entire row, column, or block.

With practice, however, the Hidden Single becomes one of the most natural techniques. Many moves in easy and medium Sudoku puzzles come precisely from asking where a certain number can go in a specific area.

It is not useful to think of one technique as “better” than the other. They are complementary and are often used together.

To practice Naked Single and Hidden Single together, you can follow a simple method.

First check the cells with few candidates. If you find a cell with only one candidate, you have a Naked Single. Then move on to rows, columns, and blocks, checking for each missing number whether there is only one possible position. In that case, you have a Hidden Single.

After every entered number, update the candidates and start checking again. A new move can create more Naked Singles or Hidden Singles.

On Sudoku Arena, you can use hints to verify your reasoning. If the hint indicates one of the two techniques, first try to identify it yourself in the grid; then read the explanation to check whether your reasoning path was correct.

Naked Single and Hidden Single are two fundamental basic techniques.

The Naked Single is found when a cell has only one possible candidate. The Hidden Single is found when a number has only one possible position in a row, column, or block.

The main difference is the starting point: the cell in the first case, the area in the second. Learning to recognize them well lets you solve many simple Sudoku puzzles and build a solid foundation for more advanced techniques.