Looking for a puzzle with Naked Single…
Naked Single
Solve one puzzle and let the hint show the technique at work. Unlimited hints, mistakes do not end the run.
Looking for a puzzle with Naked Single…
A cell where eight of the nine digits are already blocked, so exactly one candidate remains.
A cell sees three units: its row, its column and its three-by-three box. If those units already contain eight different digits between them, only one digit can legally go in that cell. That is a naked single, and it is the simplest move in Sudoku.
It is called naked because nothing is hidden: the candidate is visible in the cell itself, and you do not have to compare it with any other cell to justify the placement.
Pick a cell in a crowded region — one where its box is nearly full and its row and column are busy. Run through the digits 1 to 9 and cross off anything that already appears in any of the three units. If you reach the end of the list with one survivor, place it.
In practice you do not scan randomly. After every placement, look at the row, the column and the box of the digit you just wrote: those three units just lost a degree of freedom, and a naked single often appears in them immediately.
Every other technique on this site — pairs, pointing candidates, X-Wings, chains — removes candidates. None of them place a digit directly. They are all worth doing because of what they eventually produce: a cell with one candidate left.
So the naked single is not the beginner move you outgrow. It is the move that every advanced deduction is aiming at.
Trusting stale pencil marks. If you wrote candidates into a cell ten placements ago and never cleaned them, a cell can look like a naked single when two of its candidates are already dead, or look full when it is actually forced. When you place a digit, erase it from the notes of every cell in the same row, column and box before moving on.