Sudoku technique: hidden singles
A hidden single is a digit that has only one possible place in a Sudoku row, column or box, even though that cell could hold other digits. Solving techniques · lesson 3 of 7.
The lesson in words
Sometimes a cell has several possible digits, but one of them has nowhere else to go in its row, column or box. That’s a hidden single.
Row 3 is missing a 5. Check each empty cell in the row for whether a 5 is allowed.
Most of row 3’s empty cells already see a 5 in their column or box.
Try it: Put the 5 in the only cell of row 3 where it fits.
Ask “where can this digit go in this row?” as well as “what can go in this cell?”. Next: pencil marks.