Pointing Pairs - Sudoku69

Pointing pairs are the first technique that makes two units talk to each other, and they clear candidates that no amount of single-unit scanning would ever touch. The idea is simple: if a digit is trapped in one line inside a box, it is banned from the rest of that line.

What the pattern is

Take a box and one digit still missing from it. Look at every cell of the box where that digit is still a candidate. If all of them lie in the same row, then whichever cell finally takes the digit, it lands in that row. The box must contain the digit somewhere, so the row's share of the digit is used up inside the box, and the digit can be deleted from that row's cells in the other two boxes.

Two such cells make a pointing pair, three make a pointing triple, and the logic is identical. The same applies to columns.

	Pointing pair	Box line reduction
Digit confined to	one line inside a box	one box inside a line
You erase from	the rest of that row or column	the rest of that box
Direction	the box points outwards along the line	the line reduces the box
Cells cleared	up to six, in two other boxes	up to six, in the same box

Both are called locked candidates, and they are worth learning as a pair because you scan for them in opposite directions.

When to look for it

Once singles and pairs stop producing anything.
On digits that appear only two or three times inside a box. Those are the ones that can be aligned.
In boxes with three or four empty cells, where alignment happens naturally.
Right after a placement in a box, which often squeezes a digit down onto one line.

How to apply it, step by step

Pick a box and list the digits it still needs.
For one digit, mark every cell of the box where it remains legal.
If those cells all share one row, the digit is locked into that row inside the box.
Erase the digit from the same row in the other two boxes.
Repeat the check for columns, then for the next digit.
Work through all nine boxes. Each successful elimination can create a hidden single somewhere on the cleared line.

A worked example

Example

Box 4 (rows 4 to 6, columns 1 to 3).
x marks a cell where 7 is still a candidate:

        c1  c2  c3
  r4     -   -   -
  r5     x   -   x
  r6     -   -   -

  Box 4 needs a 7, and the only cells left for it are in row 5.
  So the 7 of row 5 lives inside box 4.
  ->  remove 7 from r5c4, r5c5, r5c6, r5c7, r5c8 and r5c9

The two cells do not have to be next to each other, and they may hold plenty of other candidates. All that matters is that no third cell of the box can take a 7.

Common mistakes

Erasing inside the box. The pattern says nothing about the other cells of the box. Only the line outside it loses the candidate.
Missing a third cell. If the digit is also legal somewhere else in the box, off that row, there is no pointing pair. Check the whole box, not just the row.
Forgetting columns. Half of all pointing pairs are vertical, and solvers who only scan rows find half as many.
Reversing the direction. Confining a digit to one box inside a row is a different pattern with a different conclusion.

Where you meet it

Pointing pairs start to matter around medium classic Sudoku and are essential from Hard upwards, in classic and Killer alike. They are also the mental habit behind most advanced patterns: find where a digit cannot go, and the grid tells you the rest.