Hidden Singles: Finding the Only Spot for a Digit
A Hidden Single in Sudoku is a digit that has only one possible cell left within a row, column, or 3x3 box. This is a foundational solving technique that reveals a placement you can pencil in with 100% certainty. While a [Naked Single](/techniques/naked-singles) is obvious because a cell has only one candidate, a Hidden Single is less obvious because you find it by looking at the placement options for a specific digit across an entire unit. Mastering this skill is a major step in moving from beginner to intermediate solving.
Hidden Single vs. Naked Single: The Core Difference
The difference between a Hidden Single and a Naked Single is what you are looking at. A Naked Single is about a **cell**. You look at a cell and see that, after eliminating other digits, only one candidate remains. The answer is forced upon you by the cell's empty space.
A Hidden Single is about a **digit**. You choose a digit (like the number 5) and scan a row, column, or box. You are looking for how many cells within that unit can still hold that digit. If you find that only one cell can possibly contain the 5, you have found a Hidden Single, even if that cell has other candidate numbers written in it. The digit 'hides' among other possibilities in that cell.
How to Spot a Hidden Single: The Systematic Scan
Finding Hidden Singles requires a shift from scanning cells to scanning digits within units. The most reliable method is a unit-by-unit, digit-by-digit scan. Select a unit—a row is often easiest. Then, for digits 1 through 9, check which cells in that row could still contain that digit. If for any digit you find only one viable cell, you have a Hidden Single.
Repeat this process for all nine rows, then all nine columns, and finally all nine 3x3 boxes. This systematic approach ensures you don't miss any. Our hint engine finds that most players speed up significantly once they internalize this digit-focused scan. It's the same logic that underpins more advanced techniques like Hidden Pairs and Triples.
- Start your scan with rows or columns that are already nearly full, as they have fewer empty cells to check.
- If you are pencil-marking candidates, a Hidden Single appears as a cell where a particular digit's candidate appears only once in its row, column, or box.
Why Beginners Often Miss Hidden Singles
New solvers naturally focus on individual cells, asking 'What number could go here?' This cell-centric view makes Naked Singles easy to spot but can cause you to overlook Hidden Singles entirely. The Hidden Single technique requires you to ask a different question: 'Where can the number 7 go in this box?'
This digit-centric perspective is a crucial skill upgrade. It's the first major step in learning to see the board as a network of constraints. If you're new to this way of thinking, our guide on How to Solve Sudoku for Beginners builds this foundational logic step by step.
Practice with a Worked Example
Look at the top row of a puzzle. Imagine the row has the numbers: 1, 2, 3, 4, _, 6, 7, 8, _. The two empty cells are at positions 5 and 9. Now, scan for the digit 5. Check each cell in the row: cells 1-4 and 6-8 are filled, so the 5 cannot go there. The only empty cells left are 5 and 9. Check the columns and boxes for those cells. You might find that the column for cell 9 already has a 5, eliminating it as an option. Therefore, the digit 5 can only go in cell 5 of that row. That's a Hidden Single.
The cell at position 5 might have candidates for 5 and 9 written in it. The presence of the 9 doesn't change the fact that the 5 *must* go there for the row to be complete. Placing the 5 will then help you solve other parts of the grid.
Key Facts
- ▪A Hidden Single is a digit that can only be placed in one specific cell within a row, column, or 3x3 box.
- ▪You find a Hidden Single by scanning for a specific digit (1-9) across an entire unit, not by looking at a single cell.
- ▪The cell containing a Hidden Single will often have other candidate numbers written in it, which is why the digit is 'hidden.'
- ▪Missing Hidden Singles is a common reason beginners get stuck; switching to a digit-scanning mindset solves this.
- ▪Hidden Singles are logically certain placements and are just as valid as the more obvious Naked Singles.
- ▪Systematically scanning each row, column, and box for each digit 1-9 is a foolproof method to find all Hidden Singles.
- ▪Spotting Hidden Singles is a prerequisite skill for identifying more complex patterns like Hidden Pairs and Triples.
- ▪In puzzles rated Easy or Medium, finding all Hidden Singles can often solve a significant portion of the grid.
Frequently Asked Questions
What is the main difference between a Naked Single and a Hidden Single?
A Naked Single is a cell with only one possible digit. A Hidden Single is a digit that has only one possible cell in a unit. You find them by looking at different things: the cell vs. the digit.
How do I know if I've found a Hidden Single?
When scanning a row, column, or box for a specific digit (like 7), if you find only one empty cell where that digit is not blocked by the same digit in the same row, column, or box, you have a Hidden Single.
Why is it called a 'Hidden' Single?
The digit is hidden because the cell it goes in usually has other candidate numbers. You must look at the digit's placement options across the unit to reveal it, not just at the cell's candidates.
Are Hidden Singles only for easy puzzles?
No. Hidden Singles appear in puzzles of all difficulty levels. Advanced techniques often create the conditions for a Hidden Single to appear, making them a critical tool for every solver.
What should I do after placing a Hidden Single?
Update the puzzle, eliminating the placed digit from candidate lists in the same row, column, and box. This often creates new Naked Singles or reveals another Hidden Single.