Naked Quad
Four cells in a unit share only four candidates.
Learn the Naked Quad Sudoku technique: four cells limited to the same four digits lock those digits out of the rest of the unit.
How to use it
- Find four linked cells. Find four cells in one row, column, or box whose candidates are all drawn from the same four digits.
- Confirm the quad. Those four digits must fill those four cells in some order, so they cannot appear anywhere else in the unit.
- Eliminate across the unit. Remove the four digits from every other cell in the unit.