Pointing Pairs and Triples
A pointing pair (or pointing triple) is an interaction between a box and a line. When a digit's only possible cells inside a box all sit in the same row or column, that digit must end up in the box somewhere along that line — so it can be eliminated from the rest of that row or column outside the box. It's one of the most common intermediate eliminations and a staple of hard-puzzle solving.
The technique is sometimes called "locked candidates, type 1." The candidates are locked because, although you don't yet know which cell of the box holds the digit, you know it lies on one particular line. That's enough to clear that line elsewhere.
How pointing works
- the unit in focus
- the pattern
- a placement
- an elimination
Inside the top-middle box, the digit 3 can only go in two cells (amber) — and both lie in the same row. We don't know which of the two will be the 3, but we know the box's 3 is somewhere on that row. Since that row can contain only one 3 in total, no cell elsewhere in the row can be a 3. The candidate is removed from the cell to the left, outside the box (red).
If the digit had been confined to two cells sharing a column instead, the same logic would point down the column. Three confined cells in a line make a pointing triple; the effect is identical.
How to spot it
For each box, take a digit it still needs and look at which of the box's empty cells can hold it. If all of those cells fall in a single row, or a single column, you have a pointing pattern. Then follow that line out of the box and erase the digit from every cell it touches.
The cue to watch for is a digit whose candidates within a box are squeezed onto one line. This happens constantly on medium and hard boards, so it pays to check after every few placements.
Pointing vs. box/line reduction
Pointing pairs and box/line reduction are the two halves of the same box-line relationship, run in opposite directions. Pointing starts inside the box ("this digit is stuck on one line of the box, so clear the line") and eliminates along the line. Box/line reduction starts on the line ("this digit on the line can only be in one box, so clear the box") and eliminates inside the box. Solvers often confuse the two; the trick is to ask which unit is doing the confining.
Why it's useful
Pointing eliminations rarely solve a cell directly, but they thin out a line just enough to reveal a hidden single or set up a naked pair. On hard boards, a couple of well-spotted pointing pairs are often the difference between flow and frustration.
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