Naked Single in Sudoku: The Simplest Technique

The Naked Single is one of the simplest and most important Sudoku techniques. It is often the first technique you learn, even without knowing its name.

It occurs when a cell can contain only one possible number. All other numbers are excluded because they are already present in the same row, column, or block.

This technique is fundamental because it lets you make certain moves. It does not require complicated reasoning, only attention and an orderly check of the grid.

In this guide, we look at what a Naked Single is, how to recognize it, and how to use it correctly during a game.

A Naked Single is an empty cell with only one possible candidate remaining.

In classic Sudoku, each cell must contain a number from 1 to 9. To understand which numbers are possible, you need to exclude the ones already present in the cell's row, column, and block.

If, after these exclusions, only one number remains, that cell must contain exactly that number. There are no alternatives.

It is called “naked” because the correct number emerges directly from the cell's candidates. It is not hidden in a broader reasoning pattern: you simply look at that cell and see that it has only one possibility.

In a classic 9×9 Sudoku, a cell has only one possible candidate when all numbers except one have been excluded. In the simplest case, these exclusions come from the numbers already present in the same row, column, and block.

For example, imagine an empty cell. Its row already contains 1, 2, and 4. Its column contains 3, 5, and 8. Its block contains 6 and 9.

At this point, the excluded numbers are 1, 2, 3, 4, 5, 6, 8, and 9. Only 7 remains. So that cell is a Naked Single and can be filled with 7.

The reasoning is simple but very powerful. Every time a cell is left with only one candidate, you have found a safe move.

To recognize a Naked Single, you need to check the empty cells and see how many candidates remain.

If you use notes, a Naked Single is easy to spot: it is a cell where only one small number appears. If you do not use notes, you need to check mentally by observing the row, column, and block.

A good method is to start from the fullest areas of the grid. Rows, columns, and blocks with many numbers already entered tend to leave fewer possibilities for empty cells.

In easy Sudoku puzzles, Naked Singles often appear one after another. Entering one number can create new Naked Singles in connected cells.

Imagine you have an empty cell with these initial candidates: 2, 4, 7, and 9.

Then you observe the grid: the same row already contains 2, the same column already contains 4, and the block already contains 9.

At this point, you can eliminate 2, 4, and 9 from the cell's candidates. Only 7 remains.

The cell must therefore be filled with 7: this is a Naked Single.

The Naked Single is fundamental because it represents the most direct form of deduction in Sudoku.

You do not need to compare many cells or recognize special patterns. You only need to understand that a cell has no alternatives. This makes it perfect for beginners, but it remains useful even in harder Sudoku puzzles.

Even in advanced grids, many techniques are specifically used to eliminate candidates. After some eliminations, a cell may be left with only one candidate and become a Naked Single.

In practice, a Naked Single is often the final result of other reasoning. That is why it is important to recognize it immediately.

The Naked Single and the Hidden Single are both ways to find a certain number, but they work differently.

In a Naked Single, you look at a cell and notice that it has only one possible candidate. The certainty comes from the cell itself.

In a Hidden Single, instead, you look at a row, column, or block and notice that a certain number can go in only one position. The cell may also have other candidates, but that number is forced because it has no alternatives in the area being considered.

So, to remember it easily: Naked Single means “one cell, one candidate”. Hidden Single means “one number, one possible position in an area”.

To practice the Naked Single, choose easy Sudoku puzzles and focus only on this technique.

At first, try using automatic candidates, so you can visually recognize cells with only one possible number. Then try doing the same reasoning with manual notes, checking row, column, and block on your own.

A good exercise is this: before entering a number, always try to explain why that cell cannot contain any other candidate.

If you can give a clear explanation, you are using the technique correctly.

The Naked Single is a basic Sudoku technique. It occurs when a cell has only one possible candidate.

To recognize it, check the cell's row, column, and block. If all numbers except one are excluded, you can enter the remaining number with certainty.

It is a simple but fundamental technique. Learning to spot it quickly helps you solve easy Sudoku puzzles and build a solid foundation for more advanced techniques.