Hidden Quad

Four digits fit only the same four cells in a unit.

Learn the Hidden Quad Sudoku technique: four digits confined to four cells let you strip every other candidate from those cells.

How to use it

1. Track four digits. Find four digits in a unit whose possible positions fall within the same four cells.
2. Confirm the hidden quad. Those four digits are locked to those four cells, no matter what other candidates are listed there.
3. Clean the cells. Remove all other candidates from the four cells.