- Box 3: the highlighted cells can hold only 5, 6, 7, and 8.
- Each of 5, 6, 7, and 8 must take one of these cells.
- Box 3: so 5, 6, 7, and 8 can go nowhere else.
Naked Quad in sudoku
A Naked Quad is four cells of one row, column or box whose notes, taken together, are only four numbers. As in a Naked Triple, no cell has to show all of them. The four numbers can be removed from the notes of every other cell of that row, column or box.
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How to spot it
Look at a row, column or box with many empty cells, and for four of them that share four numbers between them. A Naked Quad always has a hidden set on the other cells of the unit, so if a unit has few empty cells, the smaller of the two is easier to find.
Why it works
Four cells limited to four numbers must hold exactly those four. The other cells of the unit have to do without them.
On a real puzzle
This is Naked Quad on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
This Extreme puzzle calls for Naked Quad. Fill in the singles first. When no number is obvious any more, look for the pattern, or ask for the hint: each game has one.