A naked single is a cell with only one candidate. A hidden single is a digit that can only go in one cell within a house. Together, the foundation of all solving.
In Sudoku terminology, a "single" refers to a situation where a digit can be definitively placed in a cell. There are two types: naked singles (also called sole candidates) and hidden singles. Both are essential techniques that every solver must understand, and together they can solve the majority of easy and medium puzzles.
Singles represent the most direct application of Sudoku logic. When all other possibilities have been eliminated from a cell, or when a digit has nowhere else to go within a house, the placement is forced. No guessing is involved - it is pure logical deduction.
A naked single occurs when a cell has been reduced to only one possible candidate. After checking the cell's row, column, and box and eliminating all digits that already appear in those groups, if only one digit remains, it must go in that cell. The name "naked" comes from the fact that the single candidate is plainly visible.
For example, if a cell's row contains 1, 2, 4, 5, and 8, its column contains 3, 6, and 9, and its box contains 2, 7, and 5, the only digit not present in any of these groups would be the naked single for that cell. Learn the complete technique at our naked singles strategy guide.
A hidden single occurs when a specific digit can only go in one cell within a particular house (row, column, or box), even though that cell may have multiple candidates. The single is "hidden" because the cell appears to have several possibilities, but when you check where the specific digit can go within the house, there is only one valid position.
Hidden singles are often found through crosshatching - scanning rows and columns to see where a digit can fit within a box. This technique is one of the most powerful basic tools and is responsible for solving more cells than any other single technique. See our hidden singles strategy guide for detailed examples.
In practice, most solvers use a combination of naked and hidden singles as their primary solving approach. The typical workflow involves scanning the grid for hidden singles through crosshatching, then checking for naked singles in cells with few remaining candidates. This cycle repeats until no more singles can be found.
When singles are exhausted and the puzzle is not yet complete, it is time to apply more advanced techniques. The natural next step is to look for naked pairs, hidden pairs, and pointing pairs. Visit our how to play guide for a complete learning path.
Explore more concepts in the Sudoku glossary.
A single is the simplest type of Sudoku solving technique. It occurs when either a cell has only one possible candidate (naked single) or a digit has only one possible cell within a house (hidden single). Both result in definitively placing a digit.
A naked single is a cell where all candidates except one have been eliminated, making the remaining digit the only possibility. A hidden single is when a particular digit can only go in one cell within a row, column, or box, even though that cell may have other candidates listed.
Easy puzzles can almost always be solved using only naked and hidden singles. Many medium puzzles can also be solved with singles, though they may require more careful scanning. Hard and expert puzzles typically require additional techniques beyond singles.
It depends on the situation. Naked singles are easy to spot when you maintain thorough pencil marks - just look for cells with one candidate. Hidden singles are often found through crosshatching. Many solvers scan for hidden singles first since they can be found without full pencil marking.
Look at the highlighted cell. What number can fit there?
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