How to Spot Pointing Pairs Fast in Sudoku

How to spot pointing pairs fast

To spot pointing pairs fast, take one box at a time and ask, for each value still missing from it, where that value can go. If every candidate for that value inside the box lands in the same row, the value is pointing along that row; if they all share a column, it points down that column. Because the box must place the value somewhere in that line, the value is forbidden from the rest of the line outside the box. A pointing pair, or pointing triple, is one of the most reliable eliminators in the game and costs only a quick per-box glance to find.

Reading a box line by line

The scanning habit is to treat each box as a tiny grid and notice how its missing values cluster. Half-filled boxes are the best hunting ground, because a value often has just two or three legal cells left, and alignment is easy to see. Keep pointing patterns separate from their mirror image, the box-line reduction, where a value confined to a box within a line clears the rest of the box. Pointing runs from box out to a line; the reduction runs from a line back into a box. Knowing which direction you are working keeps your eliminations correct and stops you from clearing the wrong cells.

A worked pointing pair

Look at box 1, covering rows 1 to 3 and columns 1 to 3, and suppose the digit 8 can only go at r1c1 and r1c3 inside that box. Both cells sit in row 1, so box 1 will place its 8 somewhere in row 1. That makes 8 impossible in the rest of row 1 outside the box, namely r1c4 through r1c9. If r1c7 was holding 2 and 8, it now drops to a bare 2. You did not need to know which of r1c1 or r1c3 becomes the 8; the shared line was enough to clear the candidate down the entire row, which is the quiet strength of the technique.

• Check each box for a value with only two or three candidate cells.
• See whether those cells all share one row or one column.
• Remove that value from the rest of the line outside the box.
• Favor half-filled boxes, where alignment shows up fastest.
• Confirm you are pointing out to a line, not reducing into a box.

Pointing pairs in Shapedoku

Because Shapedoku gives every value a color and a shape, a value pinned to one row inside a box reads almost like a colored arrow along that line, which suits this technique perfectly. Notes with smart cleanup keeps each box honest so the alignment you see is real, not a leftover. The free solver on shapedoku.com will confirm a pointing pair and show the cells it clears if you want to double-check. If you like puzzles built around lines and direction, Space Arrows from the studio SnailPixel plays with exactly that idea, and you can sharpen your own box scanning at app.shapedoku.com whenever you have a spare minute.