The ‘Hidden Singles’ technique is essentially a basic strategy; it also relies heavily on using candidate numbers effectively. The key principle is that a candidate number should appear only once within a 3×3 block.
Let’s dive into an example. As shown in image 1 fill all the cells with candidate numbers or notes.
As shown in image 2, the next step is to carefully review the notes in a 3×3 block highlighted with the purple color. Focus on the bottom row of this block. In this row, the left cell contains the number 3 as a candidate. Since 3 does not appear elsewhere in the 3×3 block, it qualifies as a ‘Hidden Single.’ This means 3 is the only possible number for that cell.
Thus, you can remove all other notes from this cell and confidently place the number 3 as shown in image 3.
Mastering the ‘Hidden Singles’ technique simplifies the puzzle-solving process by identifying the only possible candidate for a cell, even when it’s not immediately obvious. Once you’ve grasped this technique, you can practice it by downloading our free printable Sudoku puzzle books.