Sudoku Get the app

Sudoku techniques · Medium

Hidden Single in Sudoku

A Hidden Single is a number that has only one place left to go inside a row, column or box. The cell itself may still have several candidates, which is what hides it: you find it by asking where a number can go, not what a cell can hold.

Also known as: Unique Candidate, Pinned Digit.

Download on theApp Store Get it onGoogle Play

Why it works

If a number is a candidate in only one cell of a row, column, or box, it must go there even if that cell has other candidates.

How to spot it

Switch the question around. Instead of looking at an empty cell and asking what fits, take one number and ask where it can still go in a box.

  • Pick the number that already appears most often on the board; it has the most blockers working for you.
  • Cross-hatch: trace that number's rows and columns through the box and cross off everything they hit. One survivor means a Hidden Single.
  • Boxes are easiest to scan, but the same move works along rows and columns, and beginners forget those.

Builds on

Naked Single

Worked example

These are the app's own lesson boards. Step through them the same way a hint does.

PatternAnswerContextBlockerElimination
Step 1 of 3
Hidden Single: the pattern as the app draws it during a hint.
  1. 1

    The number 1 is missing from this row. Let's see where it can go.

  2. 2

    The other empty cells in this row are each blocked from taking 1, so remove the candidate 1 from them.

  3. 3

    Now only one cell in this row still has the candidate 1, so it must go here.

Where it fits

The game looks for the easiest technique that applies before it offers you a harder one. Hidden Single sits at position 4 of 37 on that ladder, which is why puzzles that need it are graded Medium.

Learn before thisNaked SingleLearn after thisPointing Pair