Naked Single

Level: first needed in Easy puzzles. In one line: A cell where only one number is still possible.

A Naked Single is a cell that can only hold one number. Every other number from 1 to 9 already appears somewhere in the cell's row, column or box, so there is nothing left to choose. It is the most basic Sudoku move, and every puzzle, even Extreme, ends with a long run of them.

The name comes from candidates: once you write the possible numbers into each cell as notes, a naked single is a cell with just one note left, "naked" and on its own.

Example from a real puzzle

This position comes from an Extreme puzzle in Sudoku Arena, at the exact moment the Naked Single is the next logical move. Cells are named by row and column: R3C4 means row 3, column 4.

R3C4 has only one possible digit: 3. The highlighted cell R3C4 can only be 3.

Play this puzzle and look for the move yourself. It's a practice game, so it doesn't count towards your stats.

How to spot it

1. Pick an empty cell where a crowded row crosses a crowded column.
2. List the numbers already in its row, its column and its box.
3. If eight different numbers are taken, the ninth one goes in the cell.
4. With notes switched on, simply look for any cell that has a single note.

Tips

• After every number you place, look again at the cells in the same row, column and box: they each lost a candidate and may now be singles.
• Late in a puzzle naked singles come in chains. Keep going until none are left before looking for anything harder.

Next: Hidden Single · All techniques: Sudoku techniques