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.

Step 1 of 4

Quads are easy to miss, so start by isolating the most restricted cells in row 6.

Row being checked

Step 2 of 4

R6C1, R6C7, R6C8, and R6C9 contain only 2, 5, 6, and 8 between them.

Row 6Naked quad: 2, 5, 6, 8

Step 3 of 4

Candidate 6 at R6C2 is outside the quad and can be removed.

Reserved quadCandidate to remove

Step 4 of 4

R6C2 is reduced to 3, 4, or 7 while the four quad cells retain the reserved digits.

Naked quadUpdated cell

Check four low-candidate cells only after pairs and triples. The union must be exactly four digits and all cells must share one unit.