Sudoku techniques · Expert
Naked Quad in Sudoku
A Naked Quad extends the Naked Pair and Triple to four cells. Four cells in a unit pool only four candidates between them, so those four numbers fill those four cells and the rest of the unit loses all of them.
Why it works
Four cells in the same row, column, or box whose candidates together cover only four numbers keep those numbers for themselves.
How to spot it
Quads are rarely worth hunting directly; they are usually noticed from the other side.
- A unit with nine cells that has a Naked Quad also has a Hidden Quad in the other five, and the smaller of the two is easier to see.
- Start from bivalue and trivalue cells and try to build a group whose union stays at four.
- If you are scanning a unit with only five or six blanks, look for the Hidden pattern instead: it will be smaller.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
In this row, the candidates in these four cells are limited to 1, 2, 3, 4.
- 2
These four cells together must contain 1, 2, 3, 4, so we can remove the marked candidates from the other cells in this row.
- 3
Done! We've cleaned up the extra candidates and the board is simpler now.