A digit has only one place to go in a row, column, or box, even if the cell has other candidates.
Also known as: Last Remaining Cell, Unique Candidate
A Hidden Single occurs when a particular digit can only go in one cell within a row, column, or box. The cell itself might have several candidates, but for that specific unit, this is the only cell where that digit fits. Since the digit must appear somewhere in the unit, it must go in that cell.
Hidden Singles are the workhorse technique of Sudoku solving. You'll use them more than any other method. While Naked Singles are easier to understand, Hidden Singles are actually more common in practice because they let you place digits even when cells look "busy" with multiple candidates.
The name "Hidden" comes from the fact that the placement isn't obvious from looking at the cell alone. The cell might show candidates {2, 5, 7}, but if 5 can't go anywhere else in that row, then this cell must be 5. The answer is "hidden" among the other candidates.
Choose a digit, say 7, and look at a single unit (a row, column, or box). Ask yourself: "In which cells of this unit could 7 go?" If the answer is exactly one cell, you've found a Hidden Single. Place 7 in that cell.
The logic is straightforward. Every row, column, and box must contain every digit from 1 to 9 exactly once. If digit 7 is eliminated from all cells in a row except R3C8, then R3C8 must contain 7. It doesn't matter that R3C8 might also be a candidate for digits 2 and 5. The fact that 7 has no other option in that row is enough.
When you place the digit, remove it as a candidate from all cells in the same row, column, and box. This often creates new Naked Singles or Hidden Singles elsewhere. Each placement ripples outward.
The most reliable way to find Hidden Singles is systematic scanning: pick a digit and sweep across all rows, then all columns, then all boxes. Experienced solvers do this quickly by looking at where a digit is already placed and mentally blocking out rows, columns, and boxes.
Let's find where digit 1 goes in row 3. Currently, row 3 has six digits placed (8, 6, 9, 5, 7, 4) and 1 is not yet in the row.
Step 1: Identify the empty cells in row 3. Three cells are empty: R3C1, R3C5, and R3C7.
Step 2: Can 1 go in R3C1? Check column 1. R1C1 already contains 1. So R3C1 is eliminated.
Step 3: Can 1 go in R3C5? Check column 5. R7C5 already contains 1. So R3C5 is eliminated.
Step 4: Can 1 go in R3C7? Column 7 has no 1. Box 3 (top-right) has no 1. R3C7 is the only cell in row 3 where digit 1 can go. Place 1 in R3C7.
Notice that R3C7 also has candidate 3. That's fine. The Hidden Single logic doesn't require the cell to have only one candidate. It requires the digit to have only one possible cell in the unit.
Example 2: Hidden Single in a Column
Look at column 3. Where can digit 7 go? Scanning down: R1C3 has 0 (empty, check candidates), R3C3 already has 3, R5C3 has 8, R6C3 has 0 (empty), R7C3 has 5, R8C3 has 1, R9C3 has 0 (empty). After checking row/box constraints, only R4C3 can hold 7. Place 7 there.
Example 3: Hidden Single in a Box
Look at box 9 (bottom-right). Where can digit 6 go? The box contains cells in rows 7-9, columns 7-9. Checking each empty cell: R7C7 has candidate 4, R8C7 has candidate 3, and R9C7 has candidates including 6. After eliminating 6 from cells where it conflicts with the row or column, only R9C7 can hold 6.
Explore all 28 solving techniques in our complete technique guide.