Hidden Single

A Hidden Single occurs when a digit can only go in one cell within a house (row, column, or block), even though that cell may have other candidates. The digit is "hidden" among other possibilities but is forced by the constraint of its house.

Technique

1. For each house (row, column, or block), examine each digit 1-9.
2. Count how many cells in that house can contain each digit.
3. If a digit can only go in one cell within that house, place it there.
4. The cell may have multiple candidates, but within the house, only one cell can hold that specific digit.

Tips

• Blocks often reveal Hidden Singles more easily than rows or columns.
• Focus on digits that appear frequently elsewhere in the puzzle.
• Hidden Singles become more apparent after eliminations from other techniques.