Naked Quad

Basic

Four cells in a unit share candidates that are subsets of the same four digits.

How It Works

The extension of Naked Triple to four cells. When four cells in a unit have candidates spanning exactly four digits, those four digits can be eliminated from the rest of the unit.

Rare, but critical in harder puzzles.

Example

9
25
47
6
1368
3
25
47
7
4
36
15
8
268
7
45
13
9
8
27
46
9
158
4
36
27
5
3
49
25
7
378
5
16
49
2
1234
13
168
13568
38
1678
16
123468
189
6
35
27
1
146
9
58
39
3
7
28
59
4
367
2
24
58
1
5
47
16
3
258
8
37
46
4
2
19
38
5
469
6
25
17
8
Key cells of the techniqueCells where elimination is appliedCandidate to be placedEliminated candidate (crossed out)

Box 7: R7C1={1,3}, R7C2={1,6}, R8C2={3,6,8}, R9C1={1,8} → Union {1,3,6,8} → Eliminate from other cells.

Practice with a Real Puzzle

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

Expert26 givens