Step 5 · Medium
What is Cage Pointing? How to Solve Killer Sudoku
When a number must go in a cage and all its possible cells there lie in one row, column, or box, you can remove that number from the rest of that row, column, or box outside the cage.
What it is
A number that appears in every combination for a cage must go in that cage.
If all the cells in the cage that can hold that number lie in one row (column or box), the number can't go in the rest of that row outside the cage.
Example: a 2-cell cage totaling 3 in one row is filled by 1 and 2, so no other cell in that row can be 1 or 2.
The other way around: if all the places for a number in a row are inside one cage, that cage must contain the number.
How to spot it
When a small (or large) cage must contain a certain number, check whether that number's possible cells in the cage line up.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
The combinations for the 24-cage at R3C6 are 7+9, and every one of them contains 7, so 7 must go somewhere in this cage. Inside the cage, 7 can only go in R3C6 and R4C6, all in column 6. So 7 can't go in the rest of column 6 outside the cage (R1C6, R2C6, R5C6, R6C6, and R7C6).
→ Killer Sudoku Cage Combinations Chart (2–9 Cells, All 502)