2469
Techniques · Expert
Naked Quad
Four cells in a unit whose candidates together use only four digits will take those four digits, so they can be removed from the rest of the unit. It is the Naked Pair idea with four cells.
How to spot it
Rare and easy to miss. In a unit with many empty cells, look for four cells drawing from the same four digits, with no cell holding a fifth.
Example
27
269
1
3
47
8
5
49
149
137
39
458
45
478
2
6
349
48
5
38
6
2
9
37
347
1
5
23
4
29
7
6
39
1
8
7
9
1
34
8
34
6
2
5
28
6
238
29
1
5
4
39
7
3
12
269
7
45
148
159
489
246
129
4
5
38
6
138
1379
3789
23
16
8
7
345
9
2
135
34
346
In box 9, r7c9, r8c9, r9c8 and r9c9 can only hold 2, 3, 4 and 6, so 2, 3, 4 and 6 can be removed from the other cells of box 9 (Naked Quad).
the pattern cells that lose a candidate (crossed out)