Pointing

If every possible cell for a digit inside a box lies on one line, the digit can be removed from the rest of that line.

Level: Elementary · technique 4 of 7

When to use it

This is the first elimination technique to try once singles run out. It places nothing by itself, but it shrinks other cells' candidates until a new single appears.

It is easiest to see with pencil marks: look for a box where a digit's candidates sit in only two or three cells, all in one row or column. It is also called a pointing pair or pointing triple.

How to find it

1. Pick a box and a digit it is still missing.
2. Find every cell in the box where that digit can go.
3. Check whether those cells all share one row (or column).
4. If they do, remove the digit from that row's (or column's) cells outside the box.

Worked example

On the board below, look at digit 2 in box 3.

1. Find the cells of box 3 that can take a 2. The other empty cells are blocked by a 2 already on their line: r1c8 (the 2 in row 1), r1c9 (the 2 in row 1). That leaves r2c8, r3c8.
2. Those cells all lie in column 8. Whichever one it is, the 2 of box 3 will be in column 8.
3. Column 8 holds only one 2, so no cell of it outside box 3 can be 2: remove candidate 2 from r4c8, r8c8.

Common mistakes

• Removing inside the box. The eliminations happen in the other boxes along the line.
• If the cells span two lines, nothing can be removed.
• Make sure the candidates are right first; a pointing move made from a wrong candidate spoils everything after it.