Sudoku X · Medium

Hidden Single in Sudoku X

A house with one place left for a digit. The diagonals are houses too.

The idea

A hidden single is a digit with only one place left in a house. The cell may still show other candidates, but the digit has nowhere else to go.

Sudoku X has two more houses: each long diagonal holds 1 to 9 once. They are easy to forget, so they are a good place to look.

Worked examples

2 positions from real Sudoku X puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the arrows show what rules what out.

One place left in a box

Medium puzzle · Hidden Single

Look at box 3. The digit 4 must go somewhere in this box. Where can it go?

Step 1 of 4Step 2 of 4Step 3 of 4Step 4 of 4

One place left on a diagonal

Medium puzzle · Hidden Single

The lit diagonal is a house: it must hold every digit from 1 to 9 once, like a row.

Step 1 of 5Step 2 of 5Step 3 of 5Step 4 of 5Step 5 of 5

Where it fits

MediumHidden Single is a Medium technique: Medium Sudoku X puzzles are the first to need it.

The Classic Hidden Single lesson teaches it on a plain 9×9 board. The Sudoku X overview explains the rule and the levels.

Hidden Single in the other styles: Jigsaw, Thermo, Windoku, Arrow, Kropki.

Try it on a Medium Sudoku X puzzle. Next Step points it out when it is the move.

The worked examples, every step

One place left in a box

Medium Sudoku X puzzle · Hidden Single

  1. Look at box 3. The digit 4 must go somewhere in this box. Where can it go?
  2. R2C9 is the only cell in box 3 that can still take 4. Every other cell is filled or already sees a 4. Each arrow runs from a 4 to a cell it rules out.
  3. Place 4 in R2C9.
  4. R2C9 is now 4.

One place left on a diagonal

Medium Sudoku X puzzle · Hidden Single

  1. The lit diagonal is a house: it must hold every digit from 1 to 9 once, like a row.
  2. Look at the anti-diagonal (from R1C9 to R9C1). In Sudoku X it needs every digit from 1 to 9, just like a row. Where can 7 go on it?
  3. R4C6 is the only cell on the anti-diagonal that can still take 7. Every other cell is filled or already sees a 7. Each arrow runs from a 7 to a cell it rules out.
  4. Place 7 in R4C6.
  5. R4C6 is now 7.

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