Naked Single / Single Candidate Method

The most basic solution method is called Naked Single, or Single Candidate method. The concept is straightforward: identify a blank cell where only a single candidate can be placed because all other numbers are already present in the corresponding row, column and box combined.

Let’s take the following example:

Examine the candidates for the cell in row 6 and column 8, as indicated by the dark shaded cell. Notice that in the same row, the numbers 5, 6 and 9 are present, while in the same column you find 1, 3, 4, 7 and 8. Consequently, all numbers from 1 to 9, excluding 2, are eliminated as candidates. Following the Sudoku rule that a number can only appear once in a single row, column or box, it is clear that we can fill the cell with the number 2.

Consider another example where the lightly shaded box and column collectively include all the numbers except for 6. This indicates that the dark shaded cell can only be filled with the number 6.

Please note that number 7 appears in both the column and the box. This doubling is not problematic. A number can be eliminated by examining the column, row, or box individually, regardless of its presence in the other sections.

If a clear solution is not immediately apparent, use a systematic approach! Begin in one of the puzzle's corners, and assess if that cell can be utilized using this method. If not, proceed to the adjacent cell, and so on.