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Sudoku strategy guide
Hidden quads: four digits, four possible homes
A hidden quad is found when four digits appear nowhere outside the same four cells in a unit.
Key idea
Those four digits must fill the four cells, so unrelated candidates inside them are impossible.
See the move, step by step
Track digits 2, 4, 7, and 9 across row 8.
Step 1 of 4
Scan by digit frequency
Ignore the long candidate lists and count where each missing digit can occur in row 8.
Row being scanned
Step 2 of 4
Find the four-cell boundary
Digits 2, 4, 7, and 9 are confined to R8C1, R8C3, R8C5, and R8C6.
Row 8Hidden quad: 2, 4, 7, 9
Step 3 of 4
Strip every outside candidate
Remove 6 and 8 from R8C1, 6 from R8C3, and 3 from R8C5 and R8C6.
Confined quadExtra candidates to remove
Step 4 of 4
Expose the rare subset
Only the four quad digits remain across the four cells.
Hidden quadCleaned cells
Try it on today's puzzle
Hidden quads are rare. Search only after simpler subsets fail, and verify that none of the four digits appears elsewhere in the unit.
Play today