Naked Quad
Four squares share only four candidate digits; remove those digits from every other square in that unit.
What it is
Stay in one row, column or box. If four empty squares together use only four candidate digits, those digits must fill those four squares in some order. Cross them out from every other empty square in that same unit. The quad narrows the board down; it does not decide the order inside its four squares.
How to spot it
Quads are easiest to find in units where only five or six cells remain empty, because then the quad is nearly the whole unit. Look for four cells whose candidates are drawn from a small pool of digits, and check whether the union is exactly four.
Worked example
One observation at a time: find the four-square digit pool, then see which other notes it clears.
Step 1 of 4: Look at one highlighted unit
Stay in this one row or column. Only the prepared note squares matter for now.
Why it matters
Quads are uncommon, but when a unit is dense they can produce several eliminations at once. It is also worth knowing that a naked quad in a unit with five empty cells leaves the fifth cell as an immediate single.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
Are naked quads worth looking for?
Usually only on hard and expert puzzles, and mainly in units with few empty cells. Simpler techniques almost always come first.
Is there a shortcut for finding them?
Yes: if a unit has five empty cells and four of them share four digits, the fifth cell is immediately a naked single. Counting empty cells first saves time.
Put it to use
Where you will meet it
Hard grids are built around moves like this one. That is where it earns its keep.
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