Lesson:Hidden Singles
A Hidden Single is when a digit can only go in one specific cell within a unit.
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💾 Saved locallyAbout This Lesson
The Hidden Singles technique is one of the most important and frequently applicable methods in intermediate Sudoku solving. Unlike Naked Singles — where a cell has only one possible candidate left — a Hidden Single is a digit that can only be placed in one specific cell within a unit (a row, column, or 3×3 box), even though that cell may still appear to have multiple candidates.
Hidden Singles are called "hidden" because the uniqueness of the placement is not immediately obvious when you look at the cell itself. Instead, you have to survey the entire unit and observe that, for a given digit, only one position remains legally available. This makes them slightly harder to spot than Naked Singles, but once you develop the habit of scanning each unit for each digit, they become second nature.
Mastering Hidden Singles will allow you to solve most Easy and Medium puzzles from start to finish without needing any more advanced techniques. Many experienced solvers find that a combination of Naked Singles and Hidden Singles accounts for the vast majority of placements they make throughout a solving session.
How It Works — Step by Step
Step 1 — Choose a unit to examine
Pick any row, column, or 3×3 box. You are going to check whether each digit from 1 to 9 has exactly one valid position remaining within that unit. Start with units that are already well-filled, since they will have fewer empty cells and the constraint will be easier to spot.
Step 2 — Choose a digit to track
Select a digit that does not yet appear in the unit. Now look at every empty cell in the unit and ask: "Is this digit allowed here?" A cell is forbidden for a digit if that same digit already appears in the same row, the same column, or the same 3×3 box as the cell.
Step 3 — Count remaining positions
If only one empty cell in the unit can legally hold the chosen digit, that is a Hidden Single. Place the digit there immediately. If two or more cells could still hold it, move on to the next digit or the next unit.
Step 4 — Repeat systematically
Work through all nine digits in the unit, then move to the next unit. A productive solving rhythm is to sweep row by row, then column by column, then box by box. After placing a Hidden Single, that placement may create new Hidden Singles elsewhere on the board, so restart your sweep after each placement.
When to Use This Technique
Use Hidden Singles as your primary technique immediately after exhausting all Naked Singles on the board. Because Hidden Singles require you to look at an entire unit rather than a single cell, they take slightly longer to spot — but they are extremely common on Easy, Medium, and even Hard puzzles. Any time you feel stuck and cannot see an obvious Naked Single, switch to scanning each unit for Hidden Singles before reaching for more advanced methods.
Worked Examples
Imagine you are examining the middle 3×3 box of a puzzle. Five of the nine cells are already filled. You want to place the digit 7. You look at the four empty cells and check: Cell A shares a row with a 7 elsewhere — blocked. Cell B shares a column with a 7 — blocked. Cell C shares a column with another 7 — blocked. Only Cell D has no 7 in its row, column, or box. Cell D is the Hidden Single for 7 in this box.
Another common scenario arises in rows. Suppose a row has six digits placed and three empty cells. You want to find where the digit 3 goes. Two of the three empty cells sit in columns that already contain a 3. The remaining cell, even though it might accept digits 3, 5, and 8 as raw candidates, is the only cell in the row where a 3 is legal. That makes it a Hidden Single — place 3 there.
Frequently Asked Questions
What is the difference between a Hidden Single and a Naked Single?
A Naked Single is when a cell has only one candidate digit left — all others have been eliminated by row, column, and box constraints. A Hidden Single is when a digit has only one valid cell left within a unit — even though the cell itself may have multiple candidates. You discover Naked Singles by looking at the cell; you discover Hidden Singles by looking at the digit's available positions within the unit.
Can a Hidden Single appear in a row, a column, and a box at the same time?
Yes — in fact, if a digit is a Hidden Single in multiple units simultaneously, it confirms the placement even more strongly. Seeing the constraint from multiple angles helps you spot it faster and verify correctness.
How do I know when to stop looking for Hidden Singles?
Continue sweeping for Hidden Singles until you have gone through every unit (all 9 rows, all 9 columns, all 9 boxes) without finding any new placements. At that point, your puzzle either requires pencil-mark candidate tracking or a more advanced technique.
Are Hidden Singles enough to solve a Hard puzzle?
Often not on their own. Hard puzzles are typically designed to require at least Naked/Hidden Pairs or intersection techniques such as Pointing Pairs. However, Hidden Singles will always play a role even in the hardest puzzles — every placement simplifies the board and often unlocks further Hidden Singles.
Ready to test your knowledge? Try applying this technique in our Beginner-level course, Easy Sudoku puzzles or explore Sudoku Academy. Keep training to improve your solve times and master the grid!
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Apply this technique on a real puzzle from our daily or practice modes.