EXTREMEPUZZLE BOOKS

Techniques · sudoku and killer sudoku

Empty Rectangle

In one box a digit is confined to an L or cross shape. Combined with a strong link outside the box, it rules the digit out of one cell.

In the walkthroughs, every Empty Rectangle 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 Empty Rectangle is used. Each link opens the full step-by-step solution.

Extreme Sudoku 1 (70 of 150 puzzles)

Extreme Killer Sudoku 1 (32 of 150 puzzles)

Extreme Sudoku 2 (63 of 150 puzzles)

Extreme Killer Sudoku 2 (33 of 150 puzzles)

Extreme Sudoku 3 (66 of 150 puzzles)

Extreme Killer Sudoku 3 (39 of 150 puzzles)

Where it sits

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

Cover of Extreme Sudoku 1 Extreme Sudoku 1 On sale now Cover of Extreme Killer Sudoku 1 Extreme Killer Sudoku 1 On sale now Cover of Extreme Sudoku 2 Extreme Sudoku 2 On sale now Cover of Extreme Killer Sudoku 2 Extreme Killer Sudoku 2 On sale now Cover of Extreme Sudoku 3 Extreme Sudoku 3 Out 8 December 2026 Cover of Extreme Killer Sudoku 3 Extreme Killer Sudoku 3 Out 15 December 2026