A pointing pair is the first technique that eliminates instead of placing. When all of a digit’s candidates inside one box fall on a single row or column, the box has effectively claimed that line: wherever the digit lands inside the box, it uses up that row or column’s copy. The pair “points” along the line, and the digit can be erased from the rest of it.
To spot one, work box by box, digit by digit. Mark where each unsolved digit can still go inside the box — if the spots line up on one row or column, follow that line out of the box and strike the digit from every other cell on it.
It works by cases: the digit must go in one of those aligned cells. Whichever it turns out to be, the line’s quota for that digit is spent inside the box — so no cell of that line outside the box can hold it.
On the board
In the top-middle box on this board, the digit 7 has exactly two possible cells: R1C6 and R2C6 — and both sit in column 6. Whichever of the two takes the 7, column 6’s 7 is used inside that box. That lets us strike the 7 pencilled into R9C6 at the bottom of the column — and since that cell held only 7 and 8, the elimination leaves it a single 8, ready to place on the next step.
This is the exact board the app's interactive Pointing Pair demo uses — step 9 of its 59-step walkthrough, drawn here with its live pencil marks.
