How to solve a naked quad in Sudoku
A naked quad is four cells in a unit that between them hold only four candidates, which can be removed from the rest of the unit.
How it works
Four cells in a unit share only four digits between them. Those four digits will be used up inside the four cells.
So they can be removed from every other cell in the unit.
How to spot it
It is rare and easy to miss. Look in units that are almost full, where several cells have only two or three candidates.
A worked example
This is a step from a real puzzle, a moment where the next move needs a naked quad.
Struck-through digits are the ones that can be removed.
R1C8, R2C8, R3C7, R3C8 can only hold 1, 5, 6, 7 (a naked quad), so those digits can't go anywhere else in box 3. Remove them from R1C7, R2C7, R2C9.
Where it fits in Rocksled Sudoku
Puzzles that need a naked quad are never rated below Expert. Each step of this kind adds 54 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.