Naked Single
A Naked Single occurs when you focus on an empty cell and eliminate all numbers already present in its row, column, and 3×3 block, leaving only one possible candidate. While Full House focuses on ‘the entire row/column/block,’ Naked Single focuses on ‘a single cell.’ This technique works even when there are multiple empty cells in the same area.
How to Use Naked Single
Select an empty cell.
Check which numbers are in the same row.
Check which numbers are in the same column.
Check which numbers are in the same 3×3 block.
If only one number from 1–9 is not used in any of the row, column, or block, that’s your answer.
Example
Suppose a cell’s row contains 1, 3, 5, 7, its column contains 2, 4, 8, and its block contains 6. Eliminating all of these from 1–9 leaves only 9. By combining constraints from the row, column, and block, the candidate narrows down to one and the answer is determined.
Tips for Finding Naked Singles
Since checking every empty cell one by one is tedious, prioritize intersections where rows, columns, and blocks are all densely filled. Developing the habit of noting candidate numbers makes Naked Singles easier to spot. As you write out candidates, cells with only one remaining number naturally become visible.