Obvious Singles

Also known as Naked Singles.

Obvious Singles — also known as Naked Singles — is the most direct technique that relies on candidate notes rather than manually checking a cell's row, column, and box from scratch. A cell whose notes have been narrowed down to exactly one remaining candidate has only one legal value left: every other digit has been eliminated by something placed in its row, column, or box. There's no ambiguity and no reasoning required — place the single candidate that remains. Filling that cell often removes a candidate from a neighboring cell sharing its row, column, or box, which can reduce that neighbor to a single candidate too, triggering a short chain of obvious singles one after another.

How It Works

After establishing notes for all empty cells, scan for any cell with only one candidate remaining. That single candidate is the cell's only legal value.

  1. Maintain complete, accurate notes across the grid.
  2. Find any cell with exactly one note.
  3. Place that digit. Remove it from notes in the same row, column, and box.
  4. Repeat — placing one naked single often exposes others.

Naked singles can chain: filling one cell removes a candidate from a neighbor, which may reduce that neighbor to a single candidate, triggering a cascade.

Example

Below board presents an example of an obvious single on digit 7, the only candidate left in its cell.

only one candidate left — places itself

Every neighboring cell still juggles two or three candidates. The center cell does not — its notes were narrowed down to a single digit. No choice remains. Place the 7.

On Evil Boards

Naked singles are scarce at the start and may vanish entirely for long stretches of the puzzle. You will apply more advanced techniques — like hidden pairs or pointing pairs — to remove candidate digits and manufacture the conditions in which naked singles or hidden single appear.

Because it requires only a glance at each cell rather than comparing cells against each other, Obvious Singles is usually the first technique worth re-checking after any elimination elsewhere on the board.