Basic technique · 03

Naked Single

When a cell has only one possible number left, that candidate has nowhere else to go.

BeginnerCell-based

The idea

ONE CELL, ONE CANDIDATE.

A Naked Single occurs when the row, column, and box of a cell eliminate every number except one. The remaining candidate is the only value that can legally go in that cell.

The question: After eliminating what is impossible, what is left for this cell?

Walkthroughs

FOLLOW THE CANDIDATES DOWN TO ONE.

Naked single is all about one cell with one possible number.

Walkthrough

Example 01 — One candidate remains

Look at cell r6c8, it ties with 3 houses: row 6, column 8 and box 6; row 6 has number 1, 4, 5, 7; column 8 has 3, 8, 9; and box 6 has 1, 2, 4; combining them all: 1, 2, 3, 4, 5, 7, 8 and 9. the only missing one? 6. cell r6c8 can only have one candidate 6.

What to notice: Remember a cell belongs to a particular row, column and box; combining all the numbers presented in that 3 houses, if one and only one number left, that number is the solution for that cell.

Placeholder for the example 01 — one candidate remains example puzzle screenshot

Walkthrough

Example 02 — One placement unlocks another

Placing a number can remove a candidate from a neighboring cell and leave anoter Naked Single. in this example, placing number 6 in cell r6c8 reveals Naked Single 5 at cell r4c8: row 4 has number 1, 7; column 8 has 3, 8, 9; box 6 has 1, 2, 4, 6; combining them all: 1, 2, 3, 4, 6, 7 and 8, the only missing number is 5.

What to notice: The new singles appear because the surrounding information changed.

Placeholder for the example 02 — a placement creates another Naked Single example puzzle screenshot

How it works

REDUCE THE CELL TO ONE POSSIBILITY.

Naked Singles are about the candidates remaining in one specific cell. You can find them with pencil marks or by checking the numbers that already see the cell.

CHOOSE AN EMPTY CELL

Look at an unsolved cell and consider its row, column, and 3×3 box together.

ELIMINATE USED NUMBERS

Remove every number that already appears in any of the cell's three houses.

PLACE THE LAST CANDIDATE

If exactly one candidate remains, the cell is a Naked Single and that number is forced.

A little history

A NAME FOR A SIMPLE IDEA.

Naked Single is a modern descriptive name for a basic Sudoku deduction. It describes the candidate that is visible in a cell because no other candidates remain.

The technique is commonly grouped with Singles, alongside Full House and Hidden Single.

Watch & learn

SEE THE CANDIDATES DISAPPEAR.

Coming soon...

Practice

ASK WHAT IS LEFT.

Pick an empty cell and deliberately eliminate its row, column, and box numbers. Train yourself to notice when only one candidate survives.