Naked single

A naked single is the most basic deduction in Sudoku. Look at an empty cell and cross off every number already in its row, column and box. If one number is left, that’s the answer.

How to spot it

1. Pick an empty cell, ideally one in a crowded row, column or box.
2. List the numbers 1 to 9 that already appear in that row, column or box.
3. If eight of the nine appear, the ninth goes in the cell.

A real example

r3c6 sees 1, 2, 3, 4, 6, 7, 8 and 9 in its row, column and box (the highlighted cells). Only 5 is left, so 5 goes there.

Tips

• Start with rows, columns and boxes that are nearly full. They produce naked singles fastest.
• Notes make this automatic: when a cell’s notes shrink to one number, you have a naked single.

Practice on Easy puzzles