Unit Forcing Chains

Extreme

Run chains from all positions of a digit in a unit; if all agree, the conclusion is certain.

How It Works

Similar to Digit Forcing Chains but operates on a full unit basis. All possible positions of a digit in a unit are explored; if every chain from those positions agrees on a conclusion, that conclusion is certain.

Example

593
8
16
27
4
17
16
6
14
47
12
8
19
25
3
17
14
27
8
14
26
8
13
57
15
38
5
16
9
47
2
16
8
34
9
14
26
7
35
6
13
15
24
27
36
14
35
14
45
17
15
2
7
18
19
6
39
13
5
24
38
13
65
28
14
17
28
9
34
24
37
29
58
34
16
17
16
Key cells of the techniqueCells where elimination is appliedCandidate to be placedEliminated candidate (crossed out)

Digit 6 is a candidate in 3 cells of Box 4. Running chains from each → All lead to the same elimination.

Practice with a Real Puzzle

This 9×9 puzzle is solver-verified to require this technique on its solution path.

Expert25 givens