Step 6 · Medium
What is Pointing Pair (lined up inside a box)? How to Solve Killer Sudoku
When all of a digit's candidates inside a box lie in one row or column, you can remove that digit from the rest of that row or column outside the box.
What it is
If, inside a box, a digit can only go in cells that sit in one straight line (one row or one column), the digit must be somewhere on that line.
So the rest of that line, outside the box, can't have that digit.
Example: in box 1, 5 can only go in two cells of column 2 → remove 5 from column 2 in the other boxes.
How to spot it
Inside a box, check whether a digit's candidates line up in a row or column. If they do, extend that line out of the box and remove the digit there.
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.
In box 4, 8 can only go in R4C2, R5C2, and R6C2. They all sit on column 2. So box 4's 8 is somewhere on column 2, and the rest of column 2 outside the box (R7C2 and R8C2) can't be 8.