Start with the idea
Start with four cells in the same row, column, or 3x3 box. In this example, four cells in row 3 can only use 1, 3, 5, and 7 between them. Those numbers are reserved for those cells, so other cells in the row cannot use them.
Look for this pattern
Look for a small group of squares in the same row, column, or box. The group can reserve a matching set of numbers.
This empty board keeps the puzzle numbers out of the way so the pattern is easier to see.
Example
5
6
9
1
3
9
1
5
1
4
7
3
7
3
5
8
6
5
7
8
6
3
4
8
1
7
2
6
6
2
8
4
9
8
7
Look here firstLook across row 3. Four cells contain only 1, 3, 5, and 7 between them.
This technique reviews candidates instead of solving a square right away.
Those four numbers must fill those four cells, so remove 1, 3, 5, and 7 from the other cells in the row.
- Find four cells in the same row, column, or 3x3 box.
- Their notes use only four numbers total.
- Remove those numbers from the other cells in that area.
1, 3, 5, and 7 quadquad notes to remove
When to look for it
Use it on crowded Expert boards when pairs and triples almost work but a fourth cell completes the set.
How to use it
- Inspect a row, column, or box with several cells full of notes.
- Find four cells whose combined notes are exactly four digits.
- Remove those digits from all other cells in that row, column, or box.
Common mistakes
- Quads are easy to misread. Count the combined notes carefully.
- The four cells must be in the same row, column, or box.