Start with the idea
Start with four numbers in one row, column, or 3x3 box. In this example, column 9 needs 1, 2, 4, and 6, and those numbers can only fit in four cells. Those cells are reserved for those numbers, so their extra notes can be erased.
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
3
7
1
2
4
6
6
1
9
3
5
8
9
8
8
1
4
6
9
4
8
3
7
2
4
6
8
6
8
3
5
9
4
1
9
8
1
2
6
7
Look here firstLook down this column and focus on 1, 2, 4, and 6. Those four numbers are limited to the same four cells.
This technique cleans up candidates inside the four hidden cells.
Keep 1, 2, 4, and 6 in those cells, and erase the other notes from those cells.
- Scan one row, column, or 3x3 box by number.
- Find four numbers that are limited to the same four cells.
- Keep those numbers there and remove the extra notes.
1, 2, 4, and 6 to keepextra notes to erase
When to look for it
Use it when a row, column, or box has many notes and four digits are limited to four shared cells.
How to use it
- Scan a row, column, or box by digit instead of by cell.
- Find four digits whose note locations are limited to the same four cells.
- Remove all other notes from those four cells.
Common mistakes
- Hidden quads are rare and noisy.
- They reduce notes inside the quad cells, not usually outside them.