Hidden Quad
When four numbers can only fit in the same four cells.
Explanation
In one row, column, or box, if four digits can appear only inside the same four cells, those cells must contain the quad. Other candidates in those cells can be removed.
Practice tip
Track candidate positions by digit. Hidden quads are easier to confirm when notes are already filled.
Example steps
- Choose one unit and mark the possible cells for each digit.
- Find four digits whose possible cells collapse to four positions.
- Keep the quad digits there and remove all other notes in those cells.