Sudoku · EN
Sudoku Hidden Singles: Worked Example
A hidden single is a digit with only one possible position in a row, column or box, even if that cell has several notes. Check every position for that digit in the unit, then place it in the sole remaining cell.
Solver-verified example
Starting grid; dots are empty cells.
Candidates before the move
- R5C7 = {4, 5, 7, 8}
Work through this deduction
- Row 5 still needs the digit 8. Check every empty cell in that row, rather than choosing one cell’s notes in isolation.
- Only row 5, column 7 permits 8; the empty cells in columns 2, 4 and 5 all rule it out.
- Even though column 7 has notes {4,5,7,8}, the row needs its 8 here. Write 8 and remove the other three notes from that cell.
- Recheck the crossing column and the 3×3 box after placing 8. A hidden single fixes a digit’s only position in a unit.
Eliminate: R5C7 ≠ 4; R5C7 ≠ 5; R5C7 ≠ 7.
Independent verification found one completion. Forcing each displayed eliminated candidate separately produced zero completions. The grid and candidate pattern explain the move; the final answer is a check.
Verified completion
Why this deduction works
The unit must contain the digit once. Choosing another digit in its only possible cell would leave no place for the required digit. A naked single concerns one candidate in a cell; a hidden single concerns one position for a digit in a unit. Check the whole unit before concluding.
Sources and background
Frequently asked questions
Does this move finish the puzzle?
No. It establishes only the stated placement or eliminations. Update affected candidates and continue with another supported deduction.
Are incomplete notes sufficient?
No. Candidates must include every digit still permitted by the row, column and box. Missing a legal candidate can create a false pattern.
