Direct Naked Single

Show the last hint page first

Loading demo...

A Direct Naked Single is a single technique that determines a placement by inspecting the digits already placed in an unfilled cell's peer cells, without writing candidates.

How to spot it

• For each unfilled cell, inspect its peer cells (the other cells in the same row, column, or box) and collect the digits already placed in them, including both givens and answers.
• If eight distinct digits from 1 to 9 have appeared and only digit x is missing, x must be placed in the unfilled cell.

How it works

• The rules of Sudoku require each digit from 1 to 9 to appear exactly once in every house (row, column, or box), with no repeats.
• When eight distinct digits have already appeared in an unfilled cell's peer cells, that cell can contain only the remaining digit x. Any other digit would create a repeated digit in the cell's row, column, or box, violating the Sudoku rules.

Examples

On the board below, examine each unfilled cell in turn. When we reach R6C5, we can see that digits 1, 2, 3, 4, 5, 7, 8, and 9 have already appeared in its peer cells, and only digit 6 is missing. According to the rules of Sudoku, 6 must be placed in R6C5.