Jigsaw · Medium

Locked Candidates in Jigsaw Sudoku

A region and a line that share cells lock a digit into the shared part.

The idea

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

A Jigsaw region can meet a row or column in anything from one to five cells, so locks come in shapes Classic never has. They work both ways: a region locks a digit into a line, and a line locks one into a region.

Worked examples

2 positions from real Jigsaw 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 region locks a digit into a line

Medium puzzle · Locked Candidates

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

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

A line locks a digit into a region

Medium puzzle · Locked Candidates

This time candidates are eliminated in the lit region.

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 Jigsaw puzzles are the first to need it.

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

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

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

The worked examples, every step

A region locks a digit into a line

Medium Jigsaw puzzle · Locked Candidates

  1. Start in the lit region and find where one digit can go.
  2. In region 6, find every cell that can still take 5.
  3. All of them (R4C7, R5C7) are also in column 7. Region 6 must have a 5 somewhere in those cells, so the rest of column 7 cannot. The arrows point to the cells that lose 5.
  4. Eliminate 5 from R8C7.
  5. Done: 5 is eliminated from R8C7.

A line locks a digit into a region

Medium Jigsaw puzzle · Locked Candidates

  1. This time candidates are eliminated in the lit region.
  2. In row 4, find every cell that can still take 2.
  3. All of them (R4C2, R4C3) are also in region 4. Row 4 must have a 2 somewhere in those cells, so the rest of region 4 cannot. The arrows point to the cells that lose 2.
  4. Eliminate 2 from R7C2.
  5. Done: 2 is eliminated from R7C2.

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