Learn / Coloring and chains
Sudoku technique · Coloring and chains · first needed in Expert puzzles
Remote pairs
A chain of cells sharing the same two candidates alternates their values. Cells seeing two opposite-parity members lose both digits.
How to spot it
Trace a connected path of identical bivalue cells.
The common mistake
An even number of cells puts opposite values at the ends. An arbitrary pair of endpoints is not enough.
Learn this first
Naked pair and strong links.
A worked example
This position comes from SudokuByte’s lesson on the Remote pairs. Gold cells hold the evidence; green cells are the pattern’s ends.
1
8
4
9
2
7
6
3
5
5
7
6
8
3
4
1
2
9
3
9
2
5
1
6
4
7
8
2
5
8
67
4
9
3
1
67
6
4
7
3
5
1
9
8
2
9
3
1
67
8
2
7
56
467
7
2
9
1
6
8
5
4
3
8
1
5
4
9
3
27
6
67
4
6
3
2
7
5
8
9
1
1
8
4
9
2
7
6
3
5
5
7
6
8
3
4
1
2
9
3
9
2
5
1
6
4
7
8
2
5
8
67
4
9
3
1
67
6
4
7
3
5
1
9
8
2
9
3
1
67
8
2
7
56
467
7
2
9
1
6
8
5
4
3
8
1
5
4
9
3
27
6
67
4
6
3
2
7
5
8
9
1
Where to look
Trace a connected path of identical bivalue cells.A chain of cells sharing the same two candidates alternates their values. Cells seeing two opposite-parity members lose both digits.
Read the evidence in this position
Follow the cells r6c4, r4c4, r4c9, r8c9. Every cell has the same pair 6, 7, and consecutive cells see one another.Why the deduction must follow
Set the first cell to either member of the pair. Each next cell must take the opposite one. With 4 cells, the endpoints have opposite values. A cell seeing both endpoints cannot contain either member of the pair.Apply the result
Erase 6 from r6c9. Erase 7 from r6c9. The pattern may remove candidates without immediately solving a cell.
Further reading: hodoku.sourceforge.net