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Techniques · Expert
Hidden Quad
When four digits can only go in the same four cells of a unit, those cells are reserved for them, and all other candidates in those cells go.
How to spot it
Appears in nearly empty units. Map where each missing digit can go; four digits confined to the same four cells form the quad.
Example
9
4
1
5
2
6
8
3
7
57
27
8
4
3
9
6
1
3
8
6
7
1
9
25
24
45
678
37
378
2
3567
1
4
58
9
2678
1379
23789
46
367
45
357
18
356
4
137
5
9
367
8
237
12
36
58
359
389
46
69
45
1
7
2
57
6
79
1
8
2
35
49
345
1
2
4
3
59
7
6
59
8
In row 5, 1, 2, 8 and 9 can only go in r5c1, r5c2, r5c3 and r5c8, so all other candidates can be removed from those cells (Hidden Quad).
the pattern cells that lose a candidate (crossed out)