A hidden single is sneaky. A cell might accept several numbers, but when you check where a specific number can go in a row, column, or box, only that cell works.
What is it?
Instead of looking at what numbers fit in a cell, flip your thinking. Pick a number and ask: where can this number go in this row, column, or box? If there is only one valid spot, place it there.
Only one cell in this box can hold 7.
Naked vs Hidden
The difference is perspective. Both find cells with only one answer, but they approach the problem differently.
Naked Single
Look at a cell, eliminate numbers, one remains.
Hidden Single
Look at a number, check where it can go, one cell fits.
How to find them
Pick a number from 1 to 9. In a row, column, or box, check which empty cells could hold that number. If only one cell is available, you found a hidden single.
- Choose a number to hunt, say 7.
- Look at a row, column, or box. Find empty cells.
- Check which empty cells can legally hold 7. If just one, place it.
Tips for finding them
- Start with numbers that appear frequently on the board.
- Boxes are often easier to scan than long rows or columns.
- When stuck, systematically check each number from 1 to 9.
Practice this flip in thinking
Hidden singles feel awkward at first because it reverses how you normally think. But once you get it, you will spot them quickly. Many puzzles that seem stuck open up when you ask "where can this number go?" instead of "what goes here?"