Naked quad
ExpertFour cells of the same row, column or box that, between them, can only hold the same four digits take those four digits. They can be removed from every other cell of the unit. It is the naked pair and triple with one more cell.
How to spot it
- With notes, look for cells of one unit with two to four candidates.
- Find four whose candidates, put together, are only four digits.
- Remove those digits from the other cells of the unit.
Why it works
Four cells need four different digits and only have four to share, so they use them all.
Example
6
7
8
3
9
6
3
8
9
5
4
5
3
1
8
7
2
2
1
7
8
4
6
5
3
9
3
8
5
2
9
7
1
2
6
7
9
6
4
9
5
3
1
2
Look at row 9.
Common mistakes
- Counting the digits of the four cells wrong: list them all before deciding.
- Missing that the other cells of the unit often form a hidden subset instead, which is quicker to see.