Windoku · Medium

Locked Candidates in Windoku

When a house can place a digit only where it meets another house, the digit is eliminated from the rest of that other house.

The idea

If every cell in a house that can take a digit also lies in a second house, the digit must go there, and the rest of the second house cannot have it.

A window meets the boxes in blocks of four, two, two and one cells, and each row or column in three, so windows lock in ways a Classic grid cannot. It works both ways round.

Worked examples

3 positions from real Windoku 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.

A box and a line, as in Classic

Medium puzzle · Locked Candidates

In box 2, find every cell that can still take 8.

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

A window locks a digit

Medium puzzle · Locked Candidates

Start in the lit window and find where one digit can go.

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

A digit locked into a window

Medium puzzle · Locked Candidates

This time candidates are eliminated in the lit window.

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

Where it fits

MediumLocked Candidates is a Medium technique: Medium Windoku puzzles are the first to need it.

The Classic Locked Candidates lesson teaches it on a plain 9×9 board. The Windoku overview explains the rule and the levels.

Locked Candidates in the other styles: Sudoku X, Jigsaw, Thermo, Arrow, Kropki.

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

The worked examples, every step

A box and a line, as in Classic

Medium Windoku puzzle · Locked Candidates

  1. In box 2, find every cell that can still take 8.
  2. All of them (R1C5, R2C5) are also in column 5. Box 2 must have an 8 somewhere in those cells, so the rest of column 5 cannot. The arrows point to the cells that lose 8.
  3. Eliminate 8 from R8C5, R9C5.
  4. Done: 8 is eliminated from R8C5, R9C5.

A window locks a digit

Medium Windoku puzzle · Locked Candidates

  1. Start in the lit window and find where one digit can go.
  2. In the top-left window, find every cell that can still take 1.
  3. All of them (R2C4, R3C4, R4C4) are also in column 4. The top-left window must have a 1 somewhere in those cells, so the rest of column 4 cannot. In Windoku a window is a house too, like a box. The arrows point to the cells that lose 1.
  4. Eliminate 1 from R9C4.
  5. Done: 1 is eliminated from R9C4.

A digit locked into a window

Medium Windoku puzzle · Locked Candidates

  1. This time candidates are eliminated in the lit window.
  2. In box 1, find every cell that can still take 6.
  3. All of them (R3C2, R3C3) are also in the top-left window. Box 1 must have a 6 somewhere in those cells, so the rest of the top-left window cannot. In Windoku a window is a house too, like a box. The arrows point to the cells that lose 6.
  4. Eliminate 6 from R2C4.
  5. Done: 6 is eliminated from R2C4.

Previous: Hidden Single · Next: Naked Pair