Naked Singles and Hidden Singles are the two foundation techniques every Sudoku solver must master. They are responsible for the vast majority of cell placements in easy and medium puzzles, and they remain critical even in expert solving. Understanding how they differ is the cleanest entry point into structured Sudoku thinking.
A Naked Single is a cell that has exactly one possible candidate remaining after eliminations. Once you have crossed off every digit that conflicts with the cell's row, column, and box, only one number is left - so that number must go there. It is the most direct deduction in all of Sudoku.
A Hidden Single is a digit that has exactly one possible cell within a row, column, or box. The cell may have many candidates listed, but only one of those cells in the unit can possibly hold the target digit. So that digit must go there, even though the cell itself does not look forced.
Naked Singles jump out at you - one cell, one number, easy to confirm. Hidden Singles take more work because you have to scan an entire unit looking for a digit that only fits in one place. Beginners typically find Naked Singles immediately and overlook Hidden Singles for weeks before the pattern clicks.
Hidden Singles first. They unlock cells that immediately create new Naked Singles. Going Naked-first wastes time because you fill in obvious cells when scanning a row would have revealed three Hidden Singles in one pass. Disciplined solvers scan unit by unit for Hidden Singles before checking individual cells.
Together, Naked and Hidden Singles solve the entire grid for easy puzzles and most of medium puzzles. Even in expert puzzles, every advanced technique ultimately produces Singles - so reliable scanning for both is the foundation that everything else rests on.
Naked Single = a cell with only one candidate. Hidden Single = a digit with only one possible cell in a unit. Same outcome (a forced placement) reached from different directions.
Both are equally critical. Hidden Singles often unlock cells faster because they scan units rather than individual cells.
Yes. Skipping Hidden Singles is the most common reason beginners get stuck on easy puzzles.
They require scanning a whole row, column, or box for a digit, rather than checking what is left in one cell. The pattern is less visual.
Most easy puzzles, yes - many can be solved with Hidden Singles alone. Some easy puzzles also need a few Naked Pairs or Pointing Pairs.
Naked Pairs, Hidden Pairs, and Pointing Pairs - the intermediate techniques that unlock medium and hard puzzles.
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