Techniques · sudoku

XY-Chain

A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.

In the walkthroughs, every XY-Chain step shades the cells of the pattern and names the cell and digit it proves, so you can see the move on the grid rather than read about it. Press “Skip to next hard step” to jump straight to it.

Worked examples

Puzzles from the books where XY-Chain is used, those where it is the key technique first. Each link opens the full step-by-step solution.

Extreme Sudoku 1 (149 of 150 puzzles)

• Puzzle 1 · Expert 5 · key technique
• Puzzle 2 · Expert 5 · key technique
• Puzzle 7 · Expert 5 · key technique
• Puzzle 8 · Expert 5 · key technique
• Puzzle 21 · Expert 5 · key technique
• Puzzle 26 · Expert 5 · key technique
• Puzzle 30 · Expert 5 · key technique
• Puzzle 32 · Expert 5 · key technique

Extreme Sudoku 2 (145 of 150 puzzles)

• Puzzle 4 · Expert 5 · key technique
• Puzzle 28 · Expert 6 · key technique
• Puzzle 35 · Expert 6 · key technique
• Puzzle 39 · Expert 6 · key technique
• Puzzle 42 · Expert 7 · key technique
• Puzzle 46 · Expert 5 · key technique
• Puzzle 47 · Expert 6 · key technique
• Puzzle 61 · Expert 6 · key technique

Extreme Sudoku 3 (145 of 150 puzzles)

• Puzzle 1 · Expert 5 · key technique
• Puzzle 6 · Expert 5 · key technique
• Puzzle 7 · Expert 5 · key technique
• Puzzle 14 · Expert 5 · key technique
• Puzzle 16 · Expert 5 · key technique
• Puzzle 18 · Expert 5 · key technique
• Puzzle 33 · Expert 6 · key technique
• Puzzle 40 · Expert 5 · key technique

Where it sits

See the whole ladder on hard sudoku techniques, or look a puzzle up by number under solutions.