The Hidden Single is one of the most fundamental and financial strategies in Sudoku. While slightly more subtle than a Naked Single, it is essential for solving almost every puzzle.
What is a Hidden Single?
A Hidden Single occurs when a specific number (candidate) appears in only one cell within a specific region (a row, a column, or a 3x3 box), even if that cell contains other candidates.
Because every row, column, and box must contain every number from 1 to 9 exactly once, if a number has only one possible location in that unit, it must be placed there.
Interactive Example
Click "Apply Logic" to see the strategy in action.
Real Example Explanation
In the grid above, look at Row 1 (the very top row). We need to place a 5. * Scan the columns intersecting with Row 1. * You will see that other cells in this row are positioned such that they cannot be a 5 (due to 5s in their respective columns or boxes, or they are already filled). * The only remaining spot for a 5 in Row 1 is the very first cell (R1C1). * Therefore, R1C1 must be 5.
How to Spot It
To find Hidden Singles, you don't look at a single cell to see what fits. Instead, you look at a number and ask: "Where can this number go in this row/column/box?"
- Scan by Number: Pick a number (e.g., 5) and scan the grid.
- Cross-Hatching: Look at a 3x3 box. detailed Check the rows and columns passing through that box. If they already contain a 5, those lines are blocked.
- Elimination: If only one cell in the box remains unblocked for the number 5, that cell is a Hidden Single.
Why "Hidden"?
It is called "Hidden" because the cell itself might have multiple candidates (notes). For example, a cell notes might read 1, 5, 9. If you only look at that cell, you haven't solved it. But if you look at the whole row and realize no other cell in that row can be a 5, then the 5 is "hiding" in that cell alongside the 1 and 9. Since 5 must go somewhere in the row, and this is the only spot, 1 and 9 are discarded, and 5 is the solution.
Strategy Tip
Always alternate between looking for Naked Singles (cells with only one note) and Hidden Singles (units with only one place for a number). Often, finding one leads to the other.