Naked quads: reserve four digits at once
A naked quad is four cells in one unit whose candidate union contains exactly four digits.
Key idea
Those four cells consume all four digits, allowing the same digits to be removed from the unit's other cells.
See the move, step by step
Use the 2, 5, 6, and 8 quad across row 6. This constructed candidate state includes earlier deductions.
Step 1 of 4
Scan a candidate-dense row
Quads are easy to miss, so start by isolating the most restricted cells in row 6.
Step 2 of 4
Count four cells and four digits
R6C1, R6C7, R6C8, and R6C9 contain only 2, 5, 6, and 8 between them.
Step 3 of 4
Remove a quad digit elsewhere
Candidate 6 at R6C2 is outside the quad and can be removed.
Step 4 of 4
Leave the remaining notes
R6C2 is reduced to 3, 4, or 7 while the four quad cells retain the reserved digits.
Try it on today's puzzle
Check four low-candidate cells only after pairs and triples. The union must be exactly four digits and all cells must share one unit.
Play today