Pointing Pair Sudoku Technique
The first technique that reasons across two different units at once: what a box knows, it tells the row.
Also called: pointing pairs and triples, locked candidates type 1, box-to-line.
What a pointing pair is
Inside one box, every remaining home for a digit falls on the same row (or the same column). The digit has to go somewhere in the box, so it has to go somewhere on that line inside the box.
That makes the rest of the line safe to clear. Wherever the digit lands, it lands in this box, so no cell on that row outside the box can hold it.
It works with two cells or three — the name says pair, but a pointing triple behaves identically.
How to spot one
- Take a box and one digit, and mark every cell in the box that could still hold it. If all the marks share a row, you have a pointing pair on that row; if they share a column, on that column.
- The pattern is common right after you place a digit in a neighbouring box, because that placement is what squeezes the candidates onto one line.
- Look for boxes with two of three rows already accounted for — the digit is forced onto the third by definition.
Worked example
| Digit 7 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|
| Row 1 | |||||||||
| Row 2 | 7 | 7 | × | × | × | × | × | × | |
| Row 3 |
7 marks a cell where the digit can still go. × marks a candidate this technique removes. Blank cells are not part of the pattern.
- Rows 1 and 3 of box 1 are blocked for 7 — either the digit is already in those rows elsewhere, or the cells are filled.
- The 7 for box 1 must therefore sit at r2c1 or r2c3. You do not know which, and it does not matter.
- Since box 1 owns the 7 for row 2, every cell in row 2 from column 4 to column 9 loses 7 as a candidate.
What it eliminates
The digit, from every cell on the shared line that lies outside the box.
When you need it
From Medium grids upward. This is usually the first technique that feels like real deduction rather than bookkeeping.
Difficulty here is set by how many clues are left in the grid: Easy starts from 46, Medium 40 and Hard 34, with Extreme and Insane stripped further still. Fewer clues is what makes the techniques below necessary — the fewer digits you start with, the deeper you have to reason.