The Hidden Single is an essential beginner strategy. It finds where a specific number can only go in one place within a row, column, or box.
A Hidden Single exists when a particular number can only be placed in one cell within a row, column, or 3x3 box. The word "hidden" comes from the fact that the cell may contain several candidate numbers - the single is hidden among them. However, when you examine the entire unit (row, column, or box), you discover that this specific number has no other valid placement.
For example, suppose you are looking at a box and the number 7 has been eliminated from all cells except one. Even if that remaining cell also has candidates 2, 5, and 7, the 7 is a Hidden Single - it must go there because no other cell in the box can contain a 7.
Hidden Singles are also called "Unique Candidates" in some Sudoku literature. For more terminology, visit our Sudoku glossary.
Hidden Singles should be used alongside Naked Singles as part of your fundamental solving routine. Together, these two techniques can solve most easy puzzles and a significant portion of medium puzzles.
In hard and expert puzzles, Hidden Singles still play a crucial role - they frequently appear after you have made eliminations using more advanced strategies. Every time you remove a candidate from a cell, check whether you have created a new Hidden Single in the affected row, column, or box. For a complete overview of solving approaches, see our strategies guide or start with how to play Sudoku.
Hidden Singles is classified as a beginner-level technique, alongside Naked Singles. Despite being beginner-friendly, it remains one of the most powerful and frequently used strategies at every level of play.
Very frequent - most placements come from singles
Frequent - core technique combined with pairs
Regular - appears after advanced eliminations
Steady - revealed by complex chain-based techniques
A Hidden Single occurs when a specific number can only be placed in one cell within a row, column, or 3x3 box, even though that cell may have multiple candidates. The number is 'hidden' among other possibilities in the cell but is uniquely determined by the constraints of its unit.
A Naked Single focuses on the cell - it has only one candidate remaining. A Hidden Single focuses on the number - a particular digit can only go in one place within a row, column, or box. Both yield a definite placement, but they approach the problem from different angles.
Use the 'cross-hatching' technique: pick a number (say 5) and scan each row, column, and box to see where that number can still be placed. If there is only one valid cell in a unit, you have found a Hidden Single. Pencil marks help make this visual.
Yes. Hidden Singles are used across all difficulty levels. In easy puzzles, they appear frequently. In harder puzzles, they often emerge after you have applied more advanced elimination techniques, revealing new Hidden Singles as a result.
It depends on personal preference, but many solvers check both simultaneously. Naked Singles are often faster to spot when cells have few candidates, while Hidden Singles are found by scanning for where a number can go within a unit. Using both together covers the most ground efficiently.
Where can the digit 5 go in row 1? Two cells already hold 4 and 8 — but the rest are empty.
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