A Hidden Single occurs when a number is the only possible candidate in a row, column, or block, even though other candidates may exist in the same cell. The hidden single is not immediately obvious, requiring you to scan the entire area.
Here's how to identify and solve a Hidden Single:
- Check each row, column, and block for numbers that appear only once as candidates.
- Place that number in the corresponding cell.
- Eliminate that number as a candidate from the row, column, and block.
- Repeat the process throughout the grid to uncover other hidden singles or start over with the Naked Single technique.
At row 6, column 6, the number one is a hidden single. It is both the only possible place for a 1 in the row and the only place for a 1 in the block. This makes it the solution for that cell.
How to spot it
Pick a digit and a unit (a row, column, or box) and ask where that digit can still go. If it fits only one cell in the unit, that cell is a Hidden Single, even if the cell shows other candidates.
When to use it
Reach for it right after Naked Singles. Cross-hatching a box, scanning rows and columns for a digit, is the fastest way to surface them.
Common mistakes
- Only checking boxes. Hidden Singles hide in rows and columns just as often.
- Overlooking a cell because it shows extra candidates; the other candidates are irrelevant once the digit has nowhere else to go.
Frequently asked questions
- What is a Hidden Single in Sudoku?
- A Hidden Single is a digit that can legally go in only one cell of a given row, column, or box, even though that cell still shows other candidates. Because the digit has no other home in the unit, it is placed there.
- How do you find a Hidden Single?
- Take one digit at a time and scan a unit. Eliminate cells where the digit cannot go because it already appears in a crossing row, column, or box. If exactly one cell remains, it is a Hidden Single.
- What is cross-hatching in Sudoku?
- Cross-hatching is scanning the rows and columns that pass through a box to find where a given digit can fit. It is the standard way to spot Hidden Singles.