Pointing Pair
A digit confined to one box line eliminates it along that line.
A pointing pair is one of the first intersection techniques you meet, and the pointing pair sudoku pattern is what lets a single box reach out and strike candidates from its neighbours. It happens when a candidate digit inside a box is confined to just one of that box's rows or columns: because the digit has to land in one of those aligned cells, it can no longer appear anywhere else along that line where it crosses into other boxes.
Worked example
In box 1 the lower two rows are already filled — R2C1 to R2C3 hold 1, 2, 3 and R3C1 to R3C3 hold 5, 6, 7 — so the box's three missing digits 4, 8 and 9 are all squeezed into its top row at R1C1, R1C2 and R1C3. Take the 4: wherever it ends up among those three cells, it is certainly somewhere in row 1, so it 'points' along that row. That lets you delete 4 as a candidate from every row-1 cell outside box 1, such as R1C4.
When to use it
Reach for a pointing pair once naked and hidden singles have dried up and you are ready to start trimming candidates between boxes. Go box by box and, for each digit the box still needs, check whether all of its possible cells fall in a single row or a single column — as digits 4, 8 and 9 do in box 1's top row above. It is easiest to spot in a box that is already two-thirds full, since its remaining digits are forced into the one empty line; the moment a digit is pinned that way, scan the rest of that row or column in the neighbouring boxes for cells you can eliminate it from.
How to use it
- Find a box-locked digit. In a 3×3 box, find a digit whose only candidate cells all sit in the same row or the same column.
- Point along the line. Since the digit must stay inside the box, it must lie on that shared row or column.
- Eliminate outside the box. Remove the digit from the rest of that row or column outside the box.
Frequently asked questions
What is a pointing pair in Sudoku?
It is a candidate that survives in only two cells of a box, and both of those cells lie in the same row or column. Since one of the two must hold the digit, the digit is guaranteed to fall on that shared line, so it can be removed from the rest of the line where it passes through other boxes. When the candidate is confined to three aligned cells instead of two — as digit 4 is across R1C1, R1C2 and R1C3 above — the same logic holds and it is usually called a pointing triple.
How do I spot a pointing pair on the grid?
Work one box at a time with pencil marks in place. For each digit the box has not yet placed, note which empty cells could still take it; if every one of those cells sits in the same row or the same column of the box, you have a pointing pair. Boxes that are already mostly filled are the richest hunting ground, because their few missing digits get crammed into the single remaining line — exactly what forces 4, 8 and 9 into row 1 of box 1 in the example.
What is the difference between a pointing pair and box/line reduction?
They use the same box-line intersection but point in opposite directions. A pointing pair starts inside a box, with a digit confined to one of the box's lines, and eliminates that digit further along the line in the other two boxes it crosses. Box/line reduction, often called claiming, starts with a digit confined to one line within a whole row or column and instead eliminates it from the rest of a single box. Ask which region does the confining — the box or the line — and that tells you which way the eliminations run.