A locked pair, triple, or quad inside a cage claims its digits, freeing the cage’s other cells.
Naked subsets work inside cages just like in classic Sudoku. If N cells of a cage can only hold the same N digits, those digits are locked to that group — remove them from the cage’s remaining cells.
Killer Sudoku keeps every rule of Classic Sudoku — fill the 9×9 grid so each row, column, and 3×3 box holds the digits 1 to 9 exactly once — and adds cages: dashed groups of cells with a small target sum, where no digit may repeat inside a cage. Puzzles usually start with zero given digits, so every deduction begins from the cage sums. That single extra rule unlocks a whole family of arithmetic techniques.
Practise this in Killer mode on Sudoku Challenge, where the in-game Next Step button walks the exact deduction taught in this lesson.
Explore every Classic and Killer technique in the complete Learning Hub, or read how to play Sudoku to get started.