namlabsWhat is a pointing pair?
Every box needs each digit once. Suppose that, inside one box, a digit can only go in two cells and both lie on the same row. Whichever one it ends up in, the digit will be in that row inside this box. So the same row cannot hold the digit anywhere outside the box. The pair "points" along the row and clears it.
Example
Where can 4 go in the top-left box? Five of its cells are filled. R1C1 and R3C1 are in column 1, which already has a 4 in R7C1. That leaves R2C2 and R2C3, both in row 2.
So the box's 4 sits in row 2, and row 2 cannot have another 4. Erase 4 from R2C4 to R2C9. That alone does not place a digit, but it often turns a nearby cell into a single.
How to find pointing pairs
- Pick a box and a digit it does not have yet.
- List the cells where the digit can go in that box.
- If they all share a row (or a column), erase the digit from that row (or column) outside the box.
It also works with three cells in a line (a pointing triple). The reverse is called box–line reduction: if a row can only hold a digit inside one box, erase it from the rest of that box.
Where it fits
Pointing pairs and naked pairs are the core of hard puzzles. When they stop working, look for patterns across several houses, starting with X-Wing.
Practise on your phone
Our free Sudoku app has hints that point out the technique you can use next, with the cells highlighted. It plays offline.
