Step 24 · Master
What is Hidden Quad? How to Solve Killer Sudoku
When four digits can only go in the same four cells of a group, you can remove all other candidates from those four cells.
What it is
Like Hidden Pairs and Triples, but with four digits. In a group, four digits can only go in the same four cells.
Those cells must hold those digits, so remove all their other candidates.
How to spot it
Collect digits with few spots and look for four that share the same four cells.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
In box 7, 1, 3, 7, and 9 can only go in the four cells R7C2, R7C3, R8C2, and R9C3. Those cells must hold 1, 3, 7, and 9, so remove their other candidates (2 and 8).